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LECTURES ON HEREDITY 


DELIVERED UNDER THE AUSPICES 
OF THE 


WASHINGTON ACADEMY 
OF SCIENCES 


WASHINGTON, D. C. 
1917 


5 


PREFATORY NOTE 


In 1916 a series of four public lectures on nutrition was given 
in Washington under the auspices of the Washington Academy 
of Sciences. These addresses were published later in the Jour- 
nal of the Academy and reprinted in collective form. This plan 
having met with approval, a series of lectures on the subject of 
heredity was arranged for the current year, and Dr. H. 8. Jen- 
nings of the Johns Hopkins University, Dr. Oscar Riddle of the 
Carnegie Institution, and Dr. W. E. Castle of Harvard Univer- 
sity were invited to address the Academy upon various phases 
of this subject. Their addresses, delivered in March and April, 
have since been published in the Journal of the Academy and 
are here collectively reprinted in conformity with the lectures 
of last year. 

Lyman J. Briaas, 


Chairman, Committee on Meetings, 


Washington Academy of Sciences. 
Washington, D. C., 
July 20, 1917. 


598510 


CONTENTS! 


Page 
Observed changes in hereditary characters in relation to evolution. Prof. 
H. S. Jennings, Johns Hopkins University, Baltimore, Maryland...... 281 


The control of the sex ratio. Dr. Oscar Riddle, Department of Experimen- 
tal Evolution, Cold Spring Harbor, New York.!....................-. 319 


The réle of selection in evolution. Prof. W. E. Castle, Harvard University, 
Gsambridge; Massachusetts), ¢..socae «ae sisicle vice fiei> sie dinve.isyene dump van eeelemettere 369 


1The lectures are reprinted from the Journal of the Washington Academy of 
Sciences, Vol. 7, 1917. 


Reprinted from the JouRNAL or TH E WASHINGTON ACADPMY OF SCIENCES 
Vol. VII, No. 10, May 19, 1917 


GENETICS.—Observed changes in hereditary characters in re- 
lation to evolution! H. 8. Jennines, Johns Hopkins Uni- 
versity. 


The problem of the method of evolution is one which the bi- 
ologist finds it impossible to leave alone, although the longer 
he works at it, the farther its solution fades into the distance. 
The central point in the problem is the appearance, nature, and 
origin of the heritable variations that arise in organisms; the 
changes that occur in the hereditary constitution. I have for 
a long time been studying the appearance of heritable variations 
in certain lower organisms. Having satisfied myself as to the 
nature of the variations that arise in the creatures that I have 
studied, I have looked about to see what other workers have 
found; and to determine whether any unified picture of the 
matter can be made. Can we bring these facts which experi- 
mental work has brought out into relation with the method of 
evolution? Can we say that they exclude any particular theory? 


1 A lecture delivered before the Washington Academy of Sciences, March 15, 
1917. 


282 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


Can we say that they leave certain views admissible? Can we 
go farther and say that they make certain views probable? 
I shall hardly be so bold as even to ask whether they establish 
any particular views, though even that has been at times affirmed. 
These questions have, of course, been raised thousands of 
times; it is only because knowledge does advance, because 
. experimental work has been enormously multiplied of late, that 
there is reason to bring them up anew. I am going to try to 
put before you the present situation as it appears to me. 
What we may call the first phase of the modern experimental 
study of variation is that which culminated in the establishment 
of the fact that most of the heritable differences observed be- 
tween closely related organisms—between the members of a 
given species, for example—are not variations in the sense of 
alterations; are not active changes in constitution, but are per- 
manent diversities; they are static, not dynamic. This discovery, 
like that of Mendelian heredity, was, as you know, made long 
ago by the Frenchman Jordan; but, as in the case of Mendelism, 
science ignored it and pursued cheerfully its false path till the 
facts were rediscovered in recent years. All thorough work 
has led directly to this result: that any species or kind of organ- 
ism is made up of a very great number of diverse stocks, differing 
from each other in minute particulars, but the diversities in- 
herited from generation to generation. This result has in recent 
years dominated all work on the occurrence of variations; on 
the effects of selection; on the method of evolution. The con- 
dition is particularly striking in organisms reproducing from a 
single parent, so that there is no mixing of stocks; I found it in 
a high degree in organisms of this sort which I studied. Thus 
the in usorian Paramecium I found to consist of a large number 
of such heritably diverse stocks, each stock showing within itself 
many variations that are not heritable.2 Dzfflugia corona, 
which I have recently been studying, shows the same condition 
in a marked degree.? As you know, a host of workers have 
found similar conditions in all sorts of organisms. It led to the 


2 See JennrinGs, 1908, 1909, 1910, 1911. (See Bibliography.) 
3 JENNINGS, 1916. (See Bibliography.) 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 283 


idea of the genotype (Johannsen), as the permanent germinal 
constitution of any given individual; it supported powerfully the 
conception of Mendelism as merely the working out of recombina- 
tions of mosaic-like parts of these permanent genotypes. The 
whole conception is in its essential nature static; alteration does 
not fit into the scheme. 

This discovery seemed to explain fully all the observed effects 
of selection within a species; but gave them a significance quite 
the reverse of what they had been supposed to have. It seemed 
to account for practically all the supposed variations that had 
been observed; they were not variations at all, in the sense of 
steps in evolution; they were mere instances of the static con- 
dition of diversity that everywhere prevails. Jordan, the devout 
original discoverer of this condition of affairs, maintained that 
it showed that organisms do not really vary; that there is no 
such process as evolution; and indeed this seems to be the direct 
logical conclusion to be drawn. In these days of plots and spies, 
the evolutionists might almost feel that the enemy had crept 
into their citadel and was blowing it up from within. 

Now, this multiplicity of diverse stocks really represents 
the actual condition of affairs, so far as it goes. Persons who 
are interested in maintaining that evolution 7s occurring, that 
selection is effective, and the like, make a very great mistake 
in denying the existence of the condition of diversity portrayed 
by the genotypists. What they must do is to accept that con- 
dition as a foundation, then show that it is not final; that it does 
not proceed to the end; that the diverse existing stocks, while 
heritably different as the genotypists maintain, may also change 
and differentiate, in ways not yet detected by their discoverers. 

But of course most of the adherents of the “orthodox genotype 
theory’”’ do not maintain, with their first representative Jordan, 
that no changes occur; that all is genetically static in organisms. 
Typically, they admit that mutations occur; that the genotype 
may at rare intervals transform, as a given chemical compound 
may transform into another and diverse compound. We all 
know the typical instances: the transforming mutations of 
Oenothera; the bud variations that show in a sudden change of 


284 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


color or form in plants; the dropping out of definite Mendelian 
units in Drosophila and elsewhere; the transformation of particu- 
lar Mendelian units into some other condition. 

So much then may serve as an outline of a prevailing theory; 
organisms forming a multitude of diverse strains with diverse 
genotypes; the genotype a mosaic of parts that are recombined 
in Mendelian inheritance; selection a mere process of isolating 
and recombining what already exists; large changes occurring 
at rare intervals, through the dropping out of bits of the mosaic, 
or through their complete chemica! transformation; evolution 
by saltations. 

Certain serious difficulties appear in this view of the matter; 
I shall mention merely two of them, for their practical results. 
One is the very existence of the minutely differing strains, 
which forms one of the main foundations for the genotype theory. 
How have these arisen? Not by large steps, not by saltations, 
for the differences between the strains go down to the very 
limits of detectibility. On the saltation theory, Jordan’s view 
that these things were created separate at the beginning seems 
the only solution. : 

Secondly, to many minds there appears to be an equally great 
difficulty in the origin by saltation of complex adaptive structures, 
such as the eye. I shall not analyze this difficulty, but merely 
point to it and to the first one mentioned, as having had the 
practical effect of keeping many investigators persistently at 
work looking for something besides saltations as a basis for 
evolution; looking for hereditary changes that would permit a 
continuity in transformation. Some-have been searching in the 
complex phenomena of biparental inheritance; here Castle is 
to be first named, and in a later lecture you will hear of the views 
to which he has been led. Others, like Prof. H. F. Osborn, have 
been searching from this point of view the paleontological records. 
Others of us have taken up the problem in uniparental repro- 
duction; it is here that my own work falls, and of this I will for 
a moment speak. 

Where reproduction is from a single parent we meet the 
problem of inheritance and variation in its simplest form; for 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 285 


there is nothing which complicates genetic problems so enor- 
mously as does the continual mixing of diverse stocks in biparental 
inheritance. In uniparental reproduction we have but one 
genotype to deal with; we can be certain that no hereditary 
characters are introduced from outside that genotype. 

To hope for results on the problem in which we are interested, 
we must resolve to carry on a sort of second degree research, as 
it were. That is, we must accept as a foundation the facts 
before discovered, as to the make-up of the species out of a great 
number of diverse stocks; as to the usual effects of selection being 
nothing save the isolation of such preexisting stocks. What 
we must do is to take a single such stock—choosing an organism 
that is most favorable for such work—then proceed to a most 
extensive and intensive study of heredity, of variation, and of 
the effects of selection for long periods within such a stock. 

Such an organism, most favorable from all points of view, I 
found in the rhizopod Difflugia corona. It has numerous dis- 
tinetive characters, all congenital; all inherited in a high degree;, 
yet varying from parent to offspring also; none of these char- 
acters changed by growth or environmental action during the 
life of the individual. 

Long continued work showed that a single strain of this animal, 
a'l derived by fission from a single parent, does differentiate 
gradual y, with the passage of generations, into many hereditar- 
ily diverse strains. The important facts about the hereditary 
variations and their appearance are the following: 

1. Hereditary variations arose in some few cases by rather 
large steps or ‘‘saltations.”’ 

2. But the immense majority of the hereditary variations 
were minute gradations. Variation is as continuous as can be 
detected. 

3. Hereditary variation occurred in many different ways, in 
many diverse characters. There was no single line of variation 
followed exclusively, nor in the overwhelming majority of cases. 

4. It gave rise to many diverse combinations of characters: 
large animals with long spines; small animals with long spines; 
large animals with short spines; small animals with short spines; 


286 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


and so on, for other sorts of combinations of other characters. 
Any set of characters might vary independently of the rest. 

5. The hereditary variations which arose were of just such a 
nature as to produce from a single strain the hereditarily different 
strains that are found in nature.! 

I judge that if the intermediate strains were killed, the two 
most diverse strains found in nature might well be classed as 
different species, although the question of what a species is 
must be left to the judgment or fancy of the individual. 

Such then were the results of my own studies as to the nature 
of hereditary variations and how they appear. How do these 
results compare with those found by other men? If we take a 
general survey, we find the following main classes of cases: 

1. First, we have the mutations of Oenothera and its relatives: 
large transformations occurring suddenly. Here is evidently 
one of the most interesting fields of genetics, but I cannot feel, 
in view of many extraordinary phenomena in this group, that 
the bearing on the main problems of genetics is yet clear. 

2. Second, we have a large miscellaneous collection of muta- 
tions observed in various classes of organisms: ‘‘bud variations,” 
dropping out of unit factors, and the like—all definite saltations, 
but not genetically fully analyzed. 

3. In Drosophila as studied by Morgan and his associates, we 
have the largest and most fully analyzed body of facts which 
we possess with respect to changes in hereditary character in 
any organism. The changes here are pictured as typical salta- 
tions; but of these I shall speak farther. 

4. In paleontology, as the results are presented in recent 
papers by Osborn,’ the evidence is for evolution by minute, 
continuous variations which follow a single definite trend. 

5. Finally we have the work in biparental inheritance from 
Castle and his associates:® this, as interpreted by Castle, gives 
evidence for continuous variation, not following a single neces- 
sary trend, but guided by external selection. 

4 The full account of this work is given in JENNINGS, 1916. (See Bible ) 

® See OsBornezE, 1912, 1915, 1916. (See Bibliography.) 


5 See Casrix, 1915 a, 1916, 1916 a, 1916 b, 1917; Castim and PxrLurps, 1914, 
etc. (See Bibliography.) 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 287 


Furthermore, we discover in our survey that there are at least 
two wel -marked controversies in flame at the present time: 

First, we have the general controversy between, on the one 
hand, those who are mutationists and adherents of the strict 
genotype view; on the other hand those who, like Castle, be- 
lieve that we observe continuous hereditary variations in the 
progress of biparental reproduction. The mutationists attempt 
to show that the apparent gradual modification of characters 
observed in breeding is in reality a mere working out of Mende- 
lian recombinations. Here we have contributions by Morgan 
(1916), Pearl (1916, 1917), MacDowell (1916), Hagedoorn (1914), 
and others on the one hand; while the full brunt of the attack 
is borne on the other side by Castle. : 

Second, we have a somewhat less lively controversy be- 
tween the genotypic mutationists and the paleontological up- 
holders of evolution by continuous variation. Echoes of this 
we find in recent publications by Osborn and by Morgan. 

Now let us look briefly into the points at issue in the contro- 
versy between the “genotypic mutationsts’’ and the upholders 
of gradual change during biparental inheritance. 

Castle finds that in rats he can, by selection, gradually in- 
crease or decrease the amount of color in the coat passing by 
continuous stages from one extreme to the other. As to this, 
he holds two main points: 

1. The change is an actual change in the hereditary character- 
istics of the stock; not a mere result of the recombination of 
Mencelian factors. This is the general and fundamental point 
at ‘ssue. 

2 More specifical y, he holds it to be an actual change in a 
sngle unit factor; this single factor changes its grade in a con- 
tinvous and quantitative manner. 

On the other side, the critics of these views maintain that the 
changes shown are not actuai alterations in the hereditary con- 
stitution at all, but are mere results of the recombinations of 
Mendelian factors. And specifically, they find a complete 
explanation of such results as those of Castle in the hypothesis 
of multiple modifying factors. 


288 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


The method in which these modifying factors are conceived to 
operate is-doubtless familiar to you: their application to Castle’s 
work with selection in rats will serve as an example. ‘There is 
conceived to be a single ‘‘main factor’’ which determines whether 
the “hooded pattern” shall or shall not be present. In addition 
to this there are a considerable number of “modifying factors” 
which, when the “hooded pattern’ is present, increase or de- 
crease the extent of pigmentation. When many of the positive 
factors of this sort are present, the rat’s coat has much pigment; 
when fewer are present the extent of pigment is less, and so on. 
The process of changing the extent of pigmentation by selection 
consists, according to this view, merely in making diverse com- 
binations of these factors, by proper crosses. 

This same explanation is applied to a great variety of cases. 
Castle had carried the war into the enemy’s country by predict- 
ing (or at least suggesting) that the so-called unit characters in 
Drosophila would be found to be modifiable through selection.’ 
Later research by MacDowell (1915), Zeleny and Mattoon (1915), 
Reeves (1916), Morgan (1917), and Sturtevant (1917) actually 
verified this prediction; it has indeed been found that the Dro- 
sophila mutations can be modified by selection. Again the 
mutationists counter the blow with their explanation of multiple 
modifying factors, which are segregated in the process of selec- 
tion; and they give some real evidence that such is actually the 
case. 

Now, into the merits of that particular question, as to whether 
the apparent effects of selection are really due to modifying 
factors in the manner set forth, I do not propose to enter. Castle 
maintains that they are not, and I doubt not that he will show 
you reason for that point of view. At this point my own dis- 
cussion will diverge from what I judge that he will be likely to 
give. What I am going to do is to abandon the ground that 
Castle would defend, proceed directly into the territory of the 
enemy, accept the conditions met there, then see where we come 
out in relation to the nature of variation, the effects of selection, 
and the method of evolution. 


7™See Casrnx, 1915, p. 39, (See Bibliography.) 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 289 


In no other organism have heritable variations been studied 
so thoroughly as in Drosophila, and no other body of men have 
been more thoroughgoing upholders of mutationism and of the 
multiple factor explanation of the effects of selection, than the 
students of Drosophila—Morgan, Sturtevant, Bridges, Dexter, 
Muller, MacDowell, and the others. We may therefore turn to 
the evidence from Drosophila’ with confidence that it will be 
presented with fairness to the mutationist point of view. We 
shall first ask (1) what we learn from the work on Drosophila 
as to the possibility of finding finely graded variations in a 
single unit character. Next we shall inquire (2) as to the re- 
lation of the assumed modifying factors to changes in hereditary 
constitution; to the nature of the effects of selection. 

1. First, then, what are the facts as to numerous finely graded 
variations in a single unit factor? Here we have certain remark- 
able data as to the eye-color of Drosophila; data that are of 
great interest with relation to the nature of evolutionary change. 
This fruit fly has normally ared eye. Some years ago a variation 
occurred by which the eye lost its color, becoming white, a typical 
mutation. Somewhat later, another variation came, by which 
the eye color became eosin. By those wonderfully ingenious 
methods which the advanced state of knowledge of the genetics 
of Drosophila have made possible, it was determined that the 
mutations white and eosin are due to changes in a particular 
part of a particular chromosome, namely, of the so-called 
X-chromosome, or chromosome I. And further, it was discovered 
that the two colors are due to different conditions of the same 
locus of the chromosome; in other words, they represent two 
different variations of the same unit. Moreover, the normal 
red color represents a third condition of that same unit. 

Somewhat later a fourth condition of this same unit was found, 
giving a,color which lies nearer the red, between the red and eosin; 
this new color was called cherry. So we have four grades or 
conditions of this single unit character. 

And now, with the minute attention paid to the distinction of 
these grades of eye color, new grades begin to come fast. In the 
November number of Genetics, Hyde (1916), adds two new grades, 


290 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


one called “‘blood,’’ near the extreme red end of the series, the 
other, called “tinged,” near the extreme white end; in fact, from 
the descriptions it requires careful examination to distinguish 
these two from red and white, respectively. Thus we have now 
six grades of this unit. And in the same number of the same 
journal, Safir (1916) adds another intermediate grade, lying 
between “tinged” and esoin; this he calls “buff.” Atl these 
seven grades are diverse conditions of the single unit factor, 
having ‘ts locus in a certain definite spot in the X-chromosome. 
Such diverse conditions of a single actor are known as multiple 
allelomorphs. 

So, up to date we know from the mutationists’ own studies of 
Drosophila that a single unit factor presents seven gradations of 
color between white and red, each gradation heritable in the 
usual Mendelian manner. These grades are the folowing: 
(1) Red; (2) blood; (8) cherry; (4) eosin; (5) buff; (6) tinged; 
(7) white. 

Three of these grades have been discovered in the ast five 
months. It would not require a bold prophet to predict that as 
the years pass we shall come to know more of these gradations, 
till all detectible differences of shade have been distinguished, and 
each shown to be inherited as a Mendelian unit. Considering 
that the work on Drosophila has been going on only about seven 
or cight years, this is remarkable progress toward a demon- 
stration that a single unit factor can present as many grades as 
can be distinguished; that the grades may give a pragmatically 
continuous series. The extreme selectionist asks only a little 
more than this. 

Besides showing that a unit factor may thus exist in numerous 
minutely differing grades, this case shows that a heritable varia- 
tion may occur so small as to be bare’y detectible. A!though the 
variations do not usually occur in this way, the case presents the 
conditions which would allow of a gradual transition from one 
extreme to the other, by means of numerous intermediate con- 
ditions. In a population in which were occurring such minute 
changes as are here shown to be possible, we could get by selec- 
tion such a continuous series of gradations as Castle describes in 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 291 


his rats. The difference in the two cases is, that in Drosophila 
variations which are large steps occur as well as do the small 
ones; and that, according to Castle’s conception of the matter, 
such minute heritable variations occur more frequently in the 
rat than in Drosophila. But on the showing of the students of 
Drosophila, there is scarcely any other difference in principle 
between what happens in Drosophila and what Castle believes 
to happen in the rat. 

2. But as we have seen, the mutationists reject the view that 
the changes in the coat color of the rat are due to alterations in 
a single unit factor; they explain this and other cases of the 
effectiveness of selection on a single character by multiple modify- 
ing factors. Accepting again their contention, the question is 
shifted to the nature of such factors. What sort of things are 
these modifying factors? What is their relation to actual changes 
in the heritable constitution of organisms? 

Our direct experimental knowledge of these “modifying fac- 
tors” is scanty. What we have comes again mainly from the 
studies of Drosophila, so that we need not suspect it of being 
colored in such a way as to favor the selectionist point of view. 
We find data as to certain known modifying factors by one of the 
workers on Drosophila, Bridges (1916), in his recent important 
paper on non-disjunction of the chromosomes. And here we 
are taken back again to the series of eye colors, and indeed to 
one particular member of the series, the middle member, called 
eosin.? Bridges tells us that he found a factor whose only 
effect was to lighten the eosin color in a fly with eosin eyes; this 
factor indeed nearly or quite turns the eosin eye white. This 
factor Bridges calls ‘‘whiting.’”’ Another factor has the effect 
of lightening the eosin color a little less, giving a sort of cream 
color; this is called ‘‘cream 6.” <A third factor dilutes the eosin 
color not so much; it is called “cream a.’’ In addition to these, 
Bridges tells us that he has discovered three other diluters of the 
eosin color; we will call them the fourth, fifth, and sixth diluters. 
And finally Bridges tells us of another factor whose only effect 


5 BripGss, 1916, p. 148 (See Bibliography.) 


292 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


is to modify eosin in the direction of a darker color: this factor 
he calls “dark.” None of these factors has any effect save on 
eosin-eyed flies. 

As you see, these things add tremendously to our gradations 
in eye color. We had already been furnished seven grades, 
from white to red; now we have seven secondary grades within 
a single one of these seven primary grades. Our list of grada- 
tions of eye color in Drosophila therefore takes now the following 
form: 


Heritable grades of eye color, Variations that give modifica- 
due.to diverse variations of a tions of the intensity of eosin, 
single unit located in Chromo- but are located in other chro- 
some I. mosomes. 


. White 1. Whiting 
. Tinged 2. Cream b 
Buff | 3. Cream a 
To 8 ae OD an 4. Fourth diluter 


NOOO WN 
eo] 
le) 
n 
5 


Cherry 5. Fifth diluter 
. Blood 6. Sixth diluter 
Red 7. Dark 


Let us hasten to add that these seven new grades are not lo- 
cated in the same unit factor as are the seven primary ones; 
their loci are in other chromosomes (or possibly in other parts of 
the same chromosome). 

Here again then we have minutely differing conditions of a 
single shade of color, brought about. by seven modifying factors. 
Bridges makes the following remark concerning them: 


A remarkably close imitation of such a multiple factor case as that 
of Castle’s hooded rats could be concocted with the chief gene eos‘n for 
reduced color, and these six diluters which by themselves produce no 
effect, but which carry the color of eosin through every dilution stage 
from the dark yellowish:pink of the eosin female to a pure white.® 


Now this is an extremely interesting statement, one that must 
arouse the keen interest of the student of the method of evolu- 
tion. In Drosophila we could get the same sort of graded results 
that Castle does with his rats, only in Drosophila this is by 


9 BripGEs, 1916, p. 149. (See Bibliography.) 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 293 


means of multiple modifying factors, whereas Castle believes 
that in the rat it is by actual alterations of the hereditary con- 
stitution! 

But what are these modifying factors? And here we come to 
the astonishing point. These modifying factors are themselves 
alterations in the hereditary constitution. Bridges leaves no doubt 
upon this point. He lists and describes them specifically as muta- 
tions; as actual changes in the hereditary material. 

Where then is the difference in principle between the condi- 
tion in Drosophila and that in the rat? In Drosophila there 
occur minute changes in the germinal material, such as to give, 
so far as our present imperfect knowledge goes, seven diverse 
grades of a color which is itself only one grade of another series 
of seven known grades. By means of these graded changes 
one could obtain, by the mutationist’s own statement, the con- 
tinuously graded results which selection actually gives. What 
more can the selectionist ask? . 

There are indeed certain differences in detail, in the notions 
entertained by the different investigators as to exactly where 
the changes occur. Castle believes that in the rat the changes 
occur all in one unit—in one chromosomal locus—giving a series 
like the primary series for eye color in Drosophla. The sup- 
porters of multiple modifying factors believe, on the other 
hand—if we are to’ accept Bridges’ account of such factors as 
typical (and it is the only account we have)—they believe, I 
say, that these minute changes have occurred in some other 
part of the germinal material. But this difference ‘s one of mere 
detail; it does not touch the fundamental question. 

This fundamental question is as to the occurrence of these 
minute changes in the hereditary constitution,:and as to the 
possibility of getting therefrom by selection various grades of a 
given external characteristic. In this, so far as I can see, there 
is complete agreement. 

Now, doubtless, there is a further diversity in the mental proc- 
esses of the two sets of men, in that the mutationist thinks of 
all these numerous grades as after all essentially discontinuous, 
as a series of steps so minute that the difference between one 


294 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


and the next one is not detectible. His opponent, on the other 
hand, perhaps thinks of the series as actually continuous. But 
the difference is not a pragmatical one; when steps become so 
minute as to be beyond detection, the question whether they 
exist becomes metaphysical. 

To put the case in brief, if the mutationists are to show that 
the existence of multiple modifying factors has any bearing on 
the general question of the effectiveness of selection, they must 
show that such factors are not themselves minute changes in 
the hereditary constitution. Not only have they made no 
attempt to do this, but in the only well-examined cases they 
state squarely that such factors are indeed alterations in cas 
hereditary constitution. 

For the inheritance of such factors as Mendelian units, of course 
absolutely nothing is required save that the location of the 
change is in a chromosome. No particular degree of magnitude; 
no unity of any other kind is required. 

But there remains one point brought out by the mutationists 
which ‘s of great importance to the student of the method of 
evolution. While they must admit, by their own account, that 
all these grades occur, so that a practically if not actually con- 
tinuous series can be formed, they of course point out that the 
changes do not occur in a continuous series. In the eye of Droso- 
phila variation may occur from red to white directly, without 
any transit onal stages; or from any grade to any other; the con- 
tinuous scale is obtained only by arranging the steps in order. 
Therefore, it is maintained, evolution may have occurred by 
such large steps, not by continuous gradations.° This is of 
course a matter deserving of serious consideration. But cer- 
tain other points must be considered also. First, the very facts 
known for Drosophila show that there is nothing to prevent a 
passage from one extreme to the other by minute changes, just 
as is held to oceur by the paleontologists and selectionists, 
although change by large steps occurs also. Secondly, in such 
cases as the eye color of Drosophila we are dealing with char- 


10 See particularly the discussion of this point in Morean, 1916, p. 7-27. 
(See Bibliography.) 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 295 


acters that are already highly developed. We know for example, 
that this particular character is formed by the cooperation of 
many separate parts of diverse chromosomes; it is a highly 
complex product of evolution. Now, we find that one or another 
‘of these parts may suddenly cease to perform its function, so 
that the red color is not completely formed; there is a sudden 
change in it; or it may disappear entirely. But is this after all 
strong evidence that in the original production of this complex 
character with its numerous underlying functional parts, there 
was the same change by sudden large steps? Indeed, is it not 
rather true that such destructive changes in a fully formed 
character could not be expected to throw light on how that 
character was built up? 

I am not unmindful of the fact that there are a few—but only 
a very few—cases in which there is indication of a positive addi- 
tion by a definite step, as when the eosin color is produced in 
white-eyed stock. But here again the underlying apparatus has 
before had the power to produce eosin and other colors. The 
white color was due to the temporary suspension of function in 
parts of the chromosomal apparatus, and it may be doubted 
whether the restoration of this function throws light on the way 
the apparatus was first developed. 

To sum up, it appears to me that the work on Drosophila is 
supplying a complete foundation, for evolution through selection 
of minute gradations. The so-called ‘‘multiple allelomorphs’’ 
show that a single unit factor may thus exist in a great number of 
grades; the ‘‘multiple modifying factors’ show that a visible 
character may be modified in the finest gradations by alterations 
in diverse parts of the germinal apparatus. The objections 
raised by the mutationists to gradual change through selection 
are breaking down as a result of the thoroughness of the muta- 
tionists’ own studies. We have already gotten completely rid 
of the notion that the germinal changes consist only in the drop- 
ping out of complete units, or that they are bound to occur in 
large steps. If the recent rate of progress is maintained, when 
such an organism as Drosophila has been studied for fifty years, 
instead of eight or nine, there will be no conceivable gradation 


296 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


of any character that will not have been detected. The only 
outstanding difficulty is the fact that large changes occur as 
well as small ones; this seems perhaps due to the fact that we are 
witnessing the disintegration of highly developed apparatus in 
place of its building up. 

In all this, except the last point, the work on Drosophila is 
in agreement with my own observation of gradual variation in 
Difflugia; with Castle’s similar results on the rat; and with the 
conclusions of paleontologists as to the gradual development of 
the characteristics of organisms in past ages. 

But there is one point in the paleontological conclusions, as 
set forth in the recent papers by Osborn, which is not in agree- 
ment with the experimental and observational results on exist- 
ing organisms; this I wish to notice briefly. Osborn sets forth 
that in following given stocks from earlier to later ages, characters 
arise from minutest beginnings, and pass by continuous gradations 
to the highly developed condition. This seems in agreement with 
experimental results, as I have tried here to set them forth. 
Further, according to Osborn, these developing characters do 
not show random variations in all directions, but follow a definite 
course, which might seem to have been in some way predeter- 
mined. And this is emphasized by the fact that the same sorts 
of characters (horns, for example) may arise independently, at 
different ages, in diverse branches of the same stock, and each 
follow in later ages the same definite course of development. 

It would appear therefore from this that there must be some 
directing tendency, some inner necessity which drives a develop- 
ing organ to follow a definite course. Evolution is characterized 
by Orthogenesis, as this phenomenon has sometimes been called. 

Now it appears to me that we do not observe this in the present 
day experimental work; by selection we can move in more than 
one direction. I do not mean that the possible variations are 
not limited by the constitution of the varying organism; they 
certainly are. But there is no indication, so far as I can see, 
that the variations push in one determinate direction only. 

Now, examining the paleontological summaries further as 
regards this (I refer to Osborn’s papers), we find certain points 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 297 


that appear to modify seriously, if they do not quite nullify, 
this conclusion that variations follow a determinate course. 

First, we do find that diverse courses are followed by given 
characters, in diverse branches of a given group; this is partic- 
ularly true of the characters of shape and proportion, which 
Osborn calls allometrons. I take it from the descriptions that 
this is likewise true at times for structural and numerical 
characters. 

A second point which Osborn sets forth is deserving of partic- 
ular attention. He states, in agreement with Waagen, that 
in any given geologic stratum, we do find, in addition to char- 
acteristics that are in the line of determinate descent, other 
variations from this line, which are of the sort that constitute 
what we call at the present time varieties; things that are like 
the diverse races of Difflugia in my own work. But, say Os- 
born and Waagen, there is a great difference in principle between 
these and the others, for those which are in the determinate line 
of progress persist into the next geologic stratum, while the 
mere varieties do not. The persistent changes were called by 
Waagen, mutations (in a sense somewhat diverse from that in 
which the word is used by de Vries). 

Osborn expresses the opinion that these ‘‘varieties’’ may be 
merely non-heritable modifications... But in our present geo- 
logic period we find just such diverging forms, in great number, 
and we find that their peculiarities are heritable; this I empha- 
sized in the introductory part of the present discussion. There 
is then no reason for supposing that these variations were not 
heritable in earlier geologic periods; there must have been many 
races heritably diverse, just as there are now; and these are what 
Waagen called varieties. 

Now since this is so, the only difference between Waagen’s 
mutations and his varieties, is that, on looking backward at 
them, we find that the former persisted and the latter did not. 
But this tells us nothing whatever about why the latter did not. 
It is perfectly possible, so far as these facts go, that it was a 


11 Ogporn, 1915, p. 225. (See Bibliography.) 


298 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


matter of selection by external conditions; many diverse stocks 
were present, on an equal footing; some were destroyed, others 
were not. 

What ground then is there for saying that the development of 
given characters followed a definite course, as if predetermined? 
The conditions described are exactly what we should require 
to find if in past ages there were many varied stocks, some of 
which were preserved by the action of natural selection. Looking 
back over the series from a later age, we are bound of course to 
find it a continuous development. If the same characteristics 
were favorable in successive ages—and there is no reason why 
they should not be so—then the same sorts of variations would 
be preserved in those successive ages; a line of development once 
begun would be continued. And if the same sort of characters 
are favorable ones in different branches of a family, then similar 
characters may well arise and follow a similar course of develop- 
ment, in the diverse branches, as Osborn states they do. But 
at the same time many other heritable variations arise, that are 
not in the line of progress, and hence are not preserved through 
selection; these are precisely the “‘varieties’ described by the 
paleontologists; the diverse races that I have described in 
Difflugia and Paramecium, and that are found to exist in all 
organisms. The conditions described by the paleontologists 
support strongly the theory of evolution by gradual change, but 
I cannot see that they tend to establish the view that variations 
show a tendency to follow a definite course, as if predetermined. 
The paleontologists appear rather to report precisely the con- 
ditions which we are bound to find if evolution occurs through 
the guidance of natural selection operating on a great number 
of diverse variations, the typical Darwinian scheme. 

There is one other point which I wish I had time to take up, 
but I have not. I will merely attempt to state in a few words 
my impression of it. This is the point made by Bateson (1914) 
in his Presidential Address before the British Association, and 
farther developed by Davenport (1916) in a recent paper: the 
proposition, namely, that since practically all observed variations 
are cases of loss and disintegration, we are driven to suppose 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 299 


that evolution has occurred by loss and disintegration. Daven- 
port combines this idea with the theory that these disintegrating 
variations follow a definite course, predetermined in large 
measure by the constitution of the disintegrating material. 

There are two points worth consideration in dealing with this 
theory. The first is one of fact; although it is true that many 
of the so-called mutations appear to be cases of loss and disinte- 
gration, yet there is no indication that this is the case in such 
effects of selection as have been described by Castle and myself; 
variations are not limited to any particular direction. Secondly, 
it appears to me that this conclusion—that because the variations 
we see are cases of loss and disintegration, therefore evolution 
must have occurred by loss and disintegration—it appears to 
me, I say, that this conclusion involves an error in logic, which 
makes it unworthy of serious consideration. The syllogism which 
it involves seems something as follows: 

1. Major premise. Evolution has occurred by progress from 
the visibly less differentiated in structure to the visibly more 
differentiated in structure. 

2. Minor premise. By observation we detect only the visibly 
less differentiated arising from the visibly more differentiated; 
we see only a process of decreasing the visible differentiation. 

3. Conclusion. The visibly more differentiated must have 
arisen from the visibly less differentiated, by decrease in the 
visible differentiation of the latter. 

The conclusion is absurd; it cannot be drawn save for the 
fact that while in the two premises we are talking of visible 
differentiation and disintegration, in the conclusion the ground 
is shifted to mean something entirely different—a sort of inner, 
invisible, purely theoretical kind of differentiation and simplicity 
and disintegration. By putting in the word visible all the way 
through, the absurdity is brought to light. All that we can 
legitimately conclude from the two premises is that we have not 
seen the process of evolution occurring. If we have seen nothing 
but loss and disintegration, this is indeed the conclusion that we 
must draw. But I believe that we cannot assert that this is all 
that we have seen. 


300 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 


To summarize then what I have obtained from experimental 
work combined with a survey of the work of others, the impression 
left is as follows: 

1. Experimental and observational study reveals that organ- 
isms are composed of great numbers of diverse stocks differing 
heritably by minute degrees. 

2. Sufficiently thorough study shows that minute heritable 
variations—so minute as to represent practically continuous 
gradations—occur in many organisms; some reproducing from a 
sing'e parent others by biparental reproduction. 

3. The same thing is reported from paleontological studies. 

4. On careful examination we find even that the same thing 
is revealed by such mutationist work as that on Drosophila; 
single characters exist in so many grades due to minute altera~ 
tions in the hereditary constitution as to form a practically 
continuous series. 

5. It is not established that heritable changes must be sudden 
large steps; while these may occur, minute heritable changes 
are more frequent. : 

6. It is not established that heritable variations follow a 
definite course as if predetermined; they occur in many directions. 

7. It is not established that all heritable changes are by 
disintegration; although many such do occur, they cannot be 
considered steps in progressive evolution from the visibly less 
complex to the visibly more complex. 

Evolution according to the typical Darwinian scheme, through 
the occurrence of many small variations and their guidance by 
natural selection, is perfectly consistent with what experimental 
and paleontological studies show us; to me it appears more 
consistent with the data than does any other theory. 


BIBLIOGRAPHY 


Bateson, W., 1914. Address of the President of the British Association for 
the Advancement of Science. Science, 40: 319-333. Bripass, C. B., 1916. Non- 
disjunction as proof of the chromosome theory of heredity. Genetics, 1: 1-52; 
107-163. Castin, W. E., 1915. Mr. Muller on the constancy of Mendelian char- 
acters. Amer. Nat., 49: 37-42. Castun, W. E., 1915a. Some experiments in 
mass selection. Amer. Nat., 49: 713-726. Castin, W. E., 1916. Can selection 
cause genetic change? Amer. Nat., 60: 248-256. Castun, W. E., 1916a. Fur- 
ther studies of piebald rats and selection, with observations on gametic coupling. 


JENNINGS: CHANGES IN HEREDITARY CHARACTERS 301 


Carnegie Inst. Wash., Pub. 241,Part 111: 161-192. Casrin,W. E., 19166. Genetics 
and Bugenics. Cambridge. Pp. 353. Castin, W.E., 1917. Piebald rats and multi- 
ple factors. Amer. Nat., 51: 102-114. Castrin, W. E., and Putuires, J. C., 1914. 
Piebald rats and selection. An experimental test of the effectiveness of selection 
and of the theory of gametic purity in Mendelian crosses. Carnegie Inst. Wash., 
Publ. 195. Pp.56. Davenport, C. B., 1916. The form of evolutionary theory that 
modern genetical research seems to favor. Amer. Nat., 50: 449-465. Hacgrpoorn, 
A. L. and Mrs. A. C., 1914. Studies on variation and selection. Zeitschr. f. ind. 
Abst. u. Vererb., 11: 145-183. Haampoorn, A. L., and Mrs. A. C., 1917. New 
light on blending and Mendelian inheritance. Amer. Nat., 51: 189-192. Hyps, 
R. R., 1916. Two new members of a sex-linked multiple (sextuple) allelomorph 
system. Genetics, 1: 535-580. Jmnnines, H. S., 1908. Heredity, variation and 
evolution in Protozoa. II. Heredity and variation of size and form in Paramecium, 
with studies of growth, environmental action and selection. Proc. Amer. Philos. 
Soc., 47: 393-546. Jmnnines, H.S., 1909. Heredity and variation in the simplest 
organisms. Amer. Nat., 43: 321-337. Jmnntnes, H. S., 1910. Experimental 
evidence on the effectiveness of selection. Amer. Nat., 44: 136-145. JENNINGS, 
H.S., 1911. Pure lines in the study of genetics in lower organisms. Amer. Nat., 
45: 79-89. Jmnnines, H.8., 1916. Heredity, variation and the results of selection 
in the uniparental reproduction of Difflugia corona. Genetics, 1: 407-534. Mac- 
Dowe tt, E. C.,1915. Bristle inheritance in Drosophila. Journ. Exper. Zool., 19: 
61-97. 1916. MacDowrtt, E. C., Piebald rats and multiple factors. Amer. 
Nat., 50: 719-742. Morean, T. H., 1916. A critique of the theory of evolution. 
Pp. 197. Princeton Univ. Press. Moraan, T. H., 1917. An examination of the 
so-called process of contamination of genes. Anat. Record, 11: 503-504. Ossorn, 
H. F., 1912. The continuous origin of certain unit characters as observed by a 
palaeontologist. Amer. Nat., 46: 185-206, 249-278. Ossporne, H. F., 1915. 
Origin of single characters as observed in fossil and living animals and plants. 
Amer. Nat., 49: 193-239. Osxorn, H. F., 1916. . Origin and evolution of life wpon 
the earth. Scientific Monthly, 3: 5-22; 170-190; 289-307 ; 313-334; 502-513; 601-614. 
Prart, Raymonp, 1915. Seventeen years selection of a character showing six- 
linked Mendelian inheritance. Amer. Nat., 49: 595-608. Prarz, Raymonp, 1916. 
Fecundity in the domestic fowl and the selection problem. Amer. Nat., 50: 89-105. 
PEARL, Raymonp, 1917. The selection problem. Amer. Nat.,51:65-91. Renves, 
Epna M., 1916. The inheritance of extra bristles in Drosophila melanogaster Meig. 
Univ. Calif. Pub. Zool., 13: 495-515. Sarir, S. R., 1916. Buff, a new allelomorph 
of white eye color in Drosophila. Genetics, 1: 584-590. Srurtevant, A. H., 1917. 
An analysis of the effect of selection on bristle number in a mutant race of Droso- 
phila. Anat. Record, 11: 504. Zeueny, C. and Marroon, E. W., 1915. The 
effect of selection upon the “‘bar-eyes’’ mutant of Drosophila. Journ. Exper. Zool., 
19: 515-529. 


Reprinted from the JouRNAL oF THE WASHINGTON ACADEMY OF SCIENCES 
Vol. VII, No. 11, June 4, 1917 


GENETICS.—The control of the sex ratio! Oscar RIDDLE, 
Department of Experimental Evolution, Cold Spring Har- 
bor, New York. 


No better way to introduce a discussion of the present sub- 
ject has occurred to me than to avail myself of the words— 
written less than three years ago—with which Professor Don- 
caster begins his very excellent book on The Determination of 
Sex: 


The question ‘Is it a boy or a girl?” is perhaps the first which is 
generally asked about the majority of mankind during the earliest 
hours of their independent existence; and the query “Will it be a 
boy or a girl?” must equally often be in the mind, even if it is less 
frequently expressed in words. This second question raises one of 
the most widely discussed problems of biology, that of the causes 
which determine whether any individual shall be male or female, and 
it suggests the still deeper question, ‘‘Why should there be male and 
female at all?” The problem of the nature and cause of Sex ranks 
in interest with that of the nature and origin of Life, and it may be 
that neither can be completely solved apart from the other. Not- 
withstanding the immense amount of brilliant speculation and re- 
search which has been devoted to the fundamental problem of Life, 
it must be admitted that hitherto no satisfactory solution has been 
found, and in some respects the question of Sex is equally obscure. 
Hardly any other problem has aroused so much speculation, and about 
few has there been such great divergence of opinion. In one direction, 
however, the last few years have seen a considerable advance, and 
we now know at least something of the causes which lead to the pro- 
duction of one or the other sex, although of the manner in which these 
causes act our ignorance is still profound. 

It is but .a short step from the question ‘Is it a boy or a girl?” to 
the further question ‘‘ Why is it a girl instead of a boy?” and yet until 
recently the answer to this latter question seemed hopelessly beyond 
our grasp, and even now, although some indications of an answer 
can be given, they do not touch the deeper problems of the real nature 
of sex. It is a remarkable thing that apart from the fundamental 
attributes of living matter—irritability, assimilation, growth, and so 
forth—no single character is so widely distributed as sex; it occurs in 
some form in every large group of animals and plants, from the highest 
to the lowest, and yet of its true nature and meaning we have hardly 
a suspicion. Other widely distributed characters have obvious func- 
tions; of the real function of sex we know nothing, and in the rare 
cases where it seems to have disappeared, the organism thrives to all 


1 A lecture delivered before the Washington Academy of Sciences, March 29, 
1917, 


320 RIDDLE: CONTROL OF SEX RATIO 


appearance just. as well withcut it. ... . . Sex, therefore, al- 
though it is almost universally found, cannot be said with certainty 
to be a necessary attribute of living things, and its real nature remains 
an apparently impenetrable mystery. 


From the statements of this rather long quotation we shall 
have occasion, during the present hour, to dissent only from the 
impression that the problem of the nature of sex offers difficul- 
ties of a magnitude comparable with those touching the origin 
of life, and that its mysteries are apparently impenetrable. 

Studies on the heredity of sex have indeed made great prog- 
ress during the last fifteen years. These studies have been con- 
cerned with. sex-linkage and with the so-called sex-chromosomes. 
Since Doncaster, whom I have ‘quoted, is one of the foremost 
workers in both of these fields of study, it is particularly signifi- 
cant that, in his opinion, all.of the results thus far obtained from 
breeding and cytology have thrown so little light on the real 
nature and meaning of sex, and that from ‘further advances in 
these fields he apparently hopes for so little. For this reason— 
and another, namely that the relation of chromosomes to hered- 
ity and sex, have probably been extensively treated in other 
lectures of this series—you will, I trust, not now require a lengthy 
survey of the relations which the chromosomes are known to bear 
to sex, but will grant most of the present hour for an examina- 
tion of the results obtained from studies of a quite different sort; 
namely, of experimental attempts to control the sex ratio, to 
learn the nature of sex, and to control the development of sex. 

It is quite necessary, however, that all experimental studies 
on the control of the sex ratio should be carried out with the 
fullest recognition of the known normal association of the chro- 
mosome numbers—particularly of the sex-chromosomes—with 
sex, if the results of such studies are to be used toward a decision 
of the question whether sex itself has been reversed or controlled. 
The all-important question concerning abnormal or unusual sex 
ratios is, of course, the question of their meaning—Has a par- 
ticular germ cell which had initial tendencies to produce one sex 
been experimentally forced to the production of the opposite 
sex? Or—a quite different thing—have the conditions of the 


RIDDLE? CONTROL! OF SEXO SA 


experiment decreased or suppressed the productién, ge liiidbesd 
the union, or modified the chromosomal ‘constitution, of (618 16f 
the types of germ cell and left the other type-normaland ‘fiaré- 
tional? These possibilities for accounting for abnortial »séx- 
ratios certainly exist and they must be squarely met by decisive 
experiment. The facts from this sphere which we shall most 
need to bear in mind while examining our series of ratios are'das 
follows: 

1. Sperm and ovum are two kinds of sex cells in respect of 
their origin from the two contrasted sexes; but, beyond this, it 
is now clear that in some animals one and the same male pro- 
duces spermatozoa of two kinds, and that these two kinds are 
not equal in their prospective sex value. Still other animal 
forms are known in which the female produces two kinds of 
eggs, having opposite prospective sex value. Most groups of in- 
sects and several mammals are known to produce two. kinds of 
sperm, while in moths (Lepidoptera) and birds the dimorphism 
of the germs exists in the eggs. 

2. In forms which reproduce in part parthenogenetically— 
such as the bee, gall fly, plant-louse, etc.—the sex is known to 
bear certain relations to chromosome number, or to maturation 
phenomena in the egg.” 

3. In wide crosses among Echinoderms, Baltzer (’09) and 
Tennent (12) have shown that when the cross is made in one of 
the two possible directions, some of the chromosomes proceeding 
from the sperms are eliminated and do not take part in embryo- 
formation. This type of chromosome behavior has been found, 
however, only in crosses of very widely separated forms. | 

Pure wild species of doves and pigeons have proved to be 
almost ideal material for obtaining highly abnormal sex-ratios, 
and for the analysis of the meaning or significance of the modi- 

2 Even if one fully concedes the “‘lethal factor’’ explanation which Morgan 
(Science, N.S. 36: 718. 1912) gives for a particular ratio (1 1; 2 2) in Droso- 
phila, a similar basis could not apply to most of the pigeon series, since here 
every ege formed in the ovary can be accounted for, and in numerous seriés every 
egg hatched: These'same facts, together with the fact that it is thesfemale 


pigeon that is heterogametic, exclude the-action of “‘assortative mating,’ as u 
cause of the sex-ratios obtained in the pigeon. 


322 RIDDLE: CONTROL OF SEX RATIO 


fied ratios. And since it has now been shown (we shall here 
attempt to demonstrate the point) that a real reversal or con- 
trol of sex has been effected in these forms, the large and dimor- 
phic ova found generally in doves and pigeons have permitted 
new lines of investigation on the nature of sex itself. As a final 
result of these studies—a result that we may very briefly indi- 
cate in advance of the presentation of the data—we believe that 
it is now reasonably clear that the two sexes are in fact the 


TABLE 1 


On THE RELATION BETWEEN ‘‘ WIDTH OF CRoss’’ AND THE SEX RatTIo 


MATINGS WIDTH OF CROSS NO. o& | No. 9 SEX-RATIO 

Columba X orientalis.............. Families 15 | 1(?2) | 15.00:1 (or 

7.50: 1) 
Alba X orientalis (spring—before 

Juul syatl) NN See emi seem ts aise oe Genera 28 10 | 2.80: 1 
Alba X orientalis (average).......... Genera 58 43 | 1.35:1 
Onentalissalbasese eset eee Genera 36 33 | 1.09: 1 
Average reciprocal crosses............ Genera 1.22:1 
Turtur X orientalis, average recipro- 

Cal"GrosseSe.: tn citemcielcaerdeter oe Species 14 18 | 0.78:1 
Unrelated orientalis.................. Same species 36 34 | 1.06:1 
in bredsorientalisseeee eae eee eee Same species 18 20 | 0.90:1 

(—) 


expression of the rate of protoplasmic activity—of metabolism— 
pitched at two different heights or levels. For the pigeon world 
the data seem quite conclusive, and when we shall have reviewed 
a part of these data we will undertake to place before you the 
experimentally modified sex ratios obtained elsewhere among 
animals, in an attempt to show that this considerable body of 
evidence supplies further confirmation, and only confirmation, 
for the modifiability of sex, and for our conclusion that the male 
sex is an expression of metabolism at a higher level, the female 
sex of metabolism at a lower or more conservative level. 


RIDDLE: CONTROL OF SEX RATIO 323 


The work which first showed the remarkable suitability of the 
wild pigeons for the analysis of the sex-problem was done by 
the late Professor Whitman who devoted many years to the 
study of these forms. Whitman obtained indisputably a pro- 
found modification of the sex-ratio, and identified in a general 
way some factors associated with the modified ratios. Whether 


\ 


TABLE 2 


Sex Ratios anp ‘‘WrptH or Cross” AS REPORTED BY VARIOUS AUTHORS 


AUTHOR CROSS = DIFFERENT a:9 
Goat X sheep Species Uo P 

Buffon?....... Dog X wolf Species (?) Sill 
Gold-finch X canary Species 16: 3 

if Various? Genera 74: 13 

Various? Species 72:18 

et teto Tetrao Species 40: 8 
Lagopus X Tetrao Genera 13: 7 

Bhllipsig: io... : Ducks Races 46: 24 
Guinea X chicken Sub-fam. 6: 0 

Pheasant X chicken Sub-fam. 12: 0 

ear Peafowl X chicken Sub-fam. 2: 0 
eee Peafowl X peafowl Genera 1: 0 
Pheasant X pheasant Genera 14: 1 

Pheasant X pheasant Species 12: 3 


Totals: s. fam. 20.0: 0; gen. 4.9: 1; sp. 4.3:1; rac. 1.9: 1 


2 Cited by Suchetet and by Guyer. 

> As summarized by Guyer from Suchutet’s studies on museum specimens. 

° Guyer’s figures refer not to breeding data, but’to the specimens available in 
various museums (British, Paris, etc.). 


the modified ratios signified a real control—a reversal—of sex 
could not at that time be definitely decided. But on this ques- 
tion he obtained three kinds of evidence, to be mentioned later, 
and all of these indicated true sex-reversal. 

Whitman showed that ‘‘width of cross” in doves and pigeons 
is of first importance in determining sex ratios in hybrid pig- 


324 RIDDLE: CONTROL OF SEX RATIO 


TABLE 3 


FERTILITY IN “ FAmILY’’ AND ‘‘GENERIC’’ CROSSES 


FAMILY CROSS GENERIC CROSS 
Common X ring Ring X turtle 
Al 6- 9-15 infert. D1 4-27-07 infert. 
A2 6-11-15 infert. D2 4-29-07 hatch. 
@B1 6-23-15 hatch. SEI 6- 2-07 hatch. 
oB2 6-25-15 hatch. SE2 6- 4-07 hatch. 
Cl 7- 1-15 infert. QF1 7-14-07 hatch. 
C2. 7- 3-15 infert. QF2 7-16-07 hatch. 
Di 7-28-15 infert. G1 8-25-07 hatch 
D2 7-30-15 infert. 2G2 8-27-07 hatch 
El 8-13-15 infert. oAl 2-12-08 hatch 
E2 ~~ 8-15-15 infert. @A2 2-14-08 hatch 
Fi 9-10-15 infert. Bl 3-18-08 hatch 
@F2 9-12-15 hatch. oB2 3-20-08 hatch 
G1 9-26-15 infert. @Cl1 4-17-08 hatch 
G2 9-28-15 infert. 9C2 4-19-08 hatch 
H1 10-10-15 infert. @D1 5-23-08 hatch 
H2 10-12-15 infert. @D2 5-25-08 hatch 
T1 10-21-15 infert. 9 El 6-26-08 hatch 
‘S12 10-23-15 hatch. 9 E2 6-28-08 hatch 
J1 11-15-15 infert. QF1 8- 9-08 hatch 
J2 11-17-15 infert. @F2 8-11-08 hatch 
K1 12-13-15 infert. G1 9-20-08 hatch 
K2 12-15-15 infert. 9G2 9-22-08 hatch. 
Ll 12-28-15 infert. SAL 3- 2-09 hatch 
L2 12-30-15 infert.? 9 A2 3- 4-09 hatch 


@ All of the succeeding 64 eggs produced by this pair—under continued ‘‘over- 
work’’—have been tested for fertility. Of these 62 jwere wholly infertile; the 
other two hatched (both are males). 


RIDDLE: CONTROL OF SEX RATIO 325 


eons and that the wider the cross the higher is the proportion of 
males. Family crosses produce, in nearly all matings, only 
male offspring. Generic crosses produce from their “‘stronger’’ 
germs—those of spring and early summer—nearly all males. If, 
however, the birds of such a generic cross be made to ‘‘ overwork 
at reproduction,” that is if their eggs are taken from them as 
soon as laid and given to other birds for incubation, then the 
same parents which in the spring threw all or nearly all male 
offspring may be made to produce all, or nearly all, female off- 
spring in late summer and autumn. At the extreme end of the 
season eggs capable of little, then of no development, are often 
found in such series. As the parent birds grow older the time of 
appearance of females, and of eggs incapable of full develop- 
ment, is reached earlier and earlier in the summer or spring. 

The relation of ‘“‘width of cross” to the sex ratio in one of the 
many species (Turtur orientalis) with which he worked is sum- 
marized’ in Table 1. Practically every gradation from the wid- 
est possible (family) cross to inbreeding shows a sex ratio in 
accordance with its position in the series.‘ The ‘‘family cross” 
shown in Table 3 has also produced only males. 

In Table 2 I have grouped according to width of cross a num- 
ber of sex ratios reported by various observers. Here again it 
is found that family crosses yield only male offspring (20 7:0 2); 
generic crosses a ratio of 4.9% : 1 92; specific crosses 4.3 : 1; 
racial crosses 1.9 : 1. The normal sex ratio, i.e., the ratio for 
any of these species mated to its own kind, is probably nearly 
1:1 or at most not higher than 1.3% :19. The method of 
collecting most of these data renders then objectionable as evi- 
dence on some important questions, and the numbers are 
small, but they certainly support the generalization that as the 
“width of the cross” is increased a relatively higher proportion 

3’ The matings included in this table were continued by the present writer; 
both earlier and later work (to 1914) are included in the summary. 

4The specific cross—T7. turtur and T. orientalis—whose ratio (0.78:1) is a 
seeming exception is in reality not an exception. One of the females used in 
this cross had been previously “‘overworked”’ and threw nearly all females as a 


consequence. For complete data see C. O. Warrman, Posthumous Works, Vol. 
II, chap. 4. The Carnegie Institution of Washington, (In press,) 


326 


‘RIDDLE: CONTROL OF SEX RATIO 


TABLE 4 


BREEDING Recorps—1914 


(St. alba o X) 


Q St. risoria 641 (old); 1918 = 42 eggs. 


QAl 
9 A2 


Ist (4) 


H1 
H2 


Ql 
912 


vd 
9 J2 
9 Kl 
QK2 


Li 
2oL2 


oM1 
9 M2 


QN1 
QN2 


JO1 
202 
oP1 

P2 


2Ql 
9 Q2 


QRI1 
oR2 
?oS1 
982 


oun 
JT2 


Series 1 
White 140 9Ul 
White, dead 2-3 9 U2 
066 g. 2d (4) = 2.243 g. 

g (4) g v1 
Inf. yolk = 1.995 g. vVv2 
Inf. yolk = 2.105 g. 

nf. yo 5g OW! 
White, killed 4-29 W2 
White 158 (?2) P 

9 XI 
Dark, killed 2-25 9X2 
White 158 % 
vy 
White 147 QY2 
White 151 
oZ1 
Broken 9Z2 
Dark (disap.?) 
Q AAI 
Dark 161(?1) 9 AA2 
White 163 
9 BB1 
White 150 9 BB2 
White, killed with ext. 
?9CCl 
Dark 150 9 CC2 
White 150 
@DD1 
Dark 149 2 DD2 
Broken 
GEE 
White 143 9 EE2 
White 137 
9 FFI 
White 154 FF2 
Dark 162 
9@GG 
Dark, dead 7-29 9 HH1 
White, dead 7-31 oHH2 
White 140 9 ill 
Dark 164 9 II2 


White 144 
White 151 


White 155 
Dark 169 


White 152 
Soft at pole 


White 161 
White 145 


Dark 161 
White, killed bef. 10-12 


White 
White, dead 10-26 


White 141 
White 146 


White 150 
White 144 


Dark, dead 11-8 
White, dead 11-10 


White 130 (21) 
White 162 (22) 


Dark 152 
White 143 


White 166 
Broken 


White dead 150 | 
White 
Dark, dead 1-9-15 


Dark, dead 11-6-15 
White, 9 da. embr. 


Ist 17 = 5 1:12 9; 2nd 17 = 4 #:13 9; last 17 = 2 f:15 Q 


RIDDLE: CONTROL OF SEX RATIO 327 


of males is produced. It may be noted in passing that this 
generalization touches the question of the nature of sexual differ- 
ence; for, studies among the most diverse animals and plants 
have afforded evidences of the ‘‘increased vigor of hybrids,” of 
what Darwin called the ‘‘good effects of crossing,” and of what 
has been observed in Mendelian breeding as the “greater vigor 
of the heterozygote.”’ The means of “increasing the vigor’’ of 
the offspring are, therefore, the very same means by which 
higher and higher proportions of males are obtained; and males, 
we have concluded from other studies, are characterized by a 
more active metabolism than that found in females. 

A glance at Table 3 will assist in making clear some of the 
advantages which the pigeons afford in the analysis of sex ratios. 
First, examining the details of the ‘‘family cross’’—it is an excep- 
tionally bad history with almost complete infertility—we note 
that only males are produced, but that a very great number of 
eggs failed completely to develop. It might be contended that 
in such a series only the male-producing eggs are fertilized, and 
for this reason only males are produced. We may fully grant 
the point; though attention should be directed to the fact that 
if this were the whole of the story it is rather remarkable that 
only 4 eggs of the 18 here shown (6 of 88 in the entire series) were 
fertilized, since it can be proved in any similar series that at least 
half of the 18 eggs (also half of the 88) were male-producing 
eggs. And a further point of interest is that while 4 of the first 
18 eggs were fertile only 2 of the last 70 eggs—produced under 
overwork, or crowded reproduction—were fertile. But to recur 
to the original point—the pigeon in any event affords an oppor- 
tunity to study the total production of the animal’s ovary; and 
this particular animal’s ovary contains all of the sexually differ- 
entiated germs. 

In the second section of Table 3 are given the details of 
a generic cross, a cross of less widely departed forms than in the 
preceding case. In these crosses practically every egg can be 
hatched and the sex of the resulting offspring learned. This was 
done in 23 of the 24 eggs here recorded. This particular record 
is one of the many made by Professor Whitman from which he 


RIDDLE: CONTROL OF SEX RATIO 


TABLE 5 


BREEDING Recorps—1914 


(St. alba @ X) @Q St. risoria 647 (young); 1913 = 18 eggs 
Series 2 
Al 1-9 Wt. yolk = 1.515 ¢. QP1 7-1 White 150 
A2 1-11 Wt. yolk = 1.595 g. Q@P2 7-3 White 15-da. embr. 
Bl 1-28 Wt. yolk = 1.590 g. Q9Q1i 7-9 White 148 
B2 1-30 Wt. yolk = 1.685 ¢. FQ2 7-11 Dark 164 
C1 2-8 Inf. yolk = 1.445 g. QR1 7-22 White 152 
C2 2-10 Broken oR2 7-24 Dark 172 
@D1 3-5 Dark 8-da. embr. 2281 8-3 White 13-da. embr. 
2D2 3-7 White $2 8-5 Broken 3-da. embr. 
SEI 3-19 Dark 167 @T1 8-12 Dark 174 
@H2 3-21 Dark 180 Q9T2 8-14 White 164 
QF1 3-29 White 154 U- 8-20 Yolk = 1.490 g. 
jF2 3-31 Dark 19 
f BLA sana lias V1 9-6 “Blood circle” 
@G1 4-8 Dark, killed 5-6 SV2 9-8 Dark 170 
G2 4-10 White, killed 5-3 
k ROAR Cs ?@W1 9-19 Dark, dead 10-16 
QHi 4-16 White 153 Q@W2 9-21 White, dead 10-14 
H2 4-18 White 1 
i ee @X1 9-80 Dark, dead 10-19 
Il 4-25 Dark 169 Q@X2 10-2 White 145 
12 427 White 154 
: fo Whe ¥i 10-29 Inf, yolk = wees 
Ji 5-5 3-da. embr. killed 9Y2 10-31 White 15-da. embr. 
J2 5-7 3-da. embr. killed es 
Z1 12-27 No dev. yolk = 1.870 ¢ 
@K1 5-14 Dark 169 i Z2 12-29 No dev. yolk = 1.925 g. 
ONG2 7 D165 White: Vo8i tye LB WM ereiiytersietsncne rntis «Gay. geen ee 
os UNS . 9641 = (170 g.) (o 170 g.) 
GLI 5-25 Dark 179 o’s (5) from lst = 155¢. 9’s (13) 
Q@L2 5-27 White 164 Bs 
= 149 g. 
g@M1 6-3 Dark 169 : o’s (3) from 2nd = 165 g. 9Q’s 
Q9M2 6-5 White 11-da. embr. ° G1) = 180 g. 
oON1 6-13 Dark 165 9 647 = (166 g.) (7165 g.) 
QN2 6-15 White 150 o’s (7) from Ist = 170g. 9’s (5) 
= 151g. 
GO1 6-22 Dark, killed 7-13 o’s (5) from 2nd= 175 g. Q’s 
O2 6-24 Wt. yolk = 1.968 (6) = 158 er. 


Ist 18 = 9 o':9; 9 


Ind 18 = 8 #:109; 


(1915 = 11 @:21 9) 


RIDDLE: CONTROL OF SEX RATIO 329 


learned the following facts: (1) Generic crosses, when not per- 
mitted to lay many eggs, produce mostly or only males.. (2) 
Such pairs, when made to lay many eggs (crowded reproduction) 
produce males predominantly from their earlier, stronger eggs, 
and predominantly or only females from the later eggs laid 
under stress of overwork. (8) From the eggs of pure wild spe- 
cies the first egg of the pair or clutch more often hatches a male; 
the second egg of the pair more often produces a female. 

These generic crosses, then, show practically full fertility and 
exclude the possibility of accounting for the abnormal sex ratio 
of either spring or autumn by any ‘“‘assortative mating” of 
germs, since the sperms by hypothesis are all alike,* and all of 
the ova are fertilized and the resulting sex of all is known. 

From series of eggs produced by generic crosses, under “‘over- 
work” it is therefore practicable to select a certain number of 
eggs from near the first and from near the last of the season, and 
have fair assurance that (in this type of mating) most if not all 
of the earlier lot are prospectively male-producing, and most or 
all of the later lot are female-producing eggs. It was this possi- 
bility that enlisted my own first efforts in the study of sex. 
And, since a single individual ovum or yolk of the pigeon is 
large enough to permit a chemical analysis—our first study was 
to determine whether possible chemical differences between the 
male and female-producing ova exist and are discoverable. 
The first analyses of the pigeon’s ova were made in April, 1911, 
and the work has been carried on continuously since that time. 
Nearly 900 individual yolks have now been analyzed. Among 
these are represented the eggs of several pure species, and of 
many kinds of hybrids. The records for the chemical composi- 
tion of the egg-yolks of a considerable number of individual fe- 
males is now complete for five consecutive years. Altogether, 
these studies, and the supplementary ones which developed out 
of them or along with them, have brought to light a number of 
facts which I can here only briefly sketch. 

Before considering the results of the analyses it may be well 
to make clear the nature of a difference which appeared as soon 


5 It is certain that the ova are sexually dimorphic. 


330° RIDDLE: CONTROL OF SEX RATIO 

as my first lots of yolk samples were placed on the balances for 
the preliminary weighings. The balances alone and at once 
showed that the mass of the yolk of the first egg of nearly all 
pairs of eggs (from pure species) was less by from (usually) 9 
per cent to 15 per cent than the mass of the yolk of the second 


TABLE 6 


Weicut oF ENTIRE EaGs, AND oF YOLKS, 1913-1915 or 9 641 and 9 647 


TOTAL 
EGGS 


EGGS. 


YOLKS 


YEAR | pRo- aoe a: 2 
DUCED Jord. no. wt. + hr.? no. wt. + hr. - 
9 641 (older) 
1913 | (42) jIst (19)= 8.532 + 253 (5) 1.903 + 153 9g: 89:10 Q 
2nd (19)= 9.221 + 16 (6) 2.153 + 203 \ 
1914 | (67) |lst (30)= 8.627 + 63 (4) 2.032 + 139 11 o: 40 9 
2nd (31)= 9.275 + 14 (5) 2.219 + 106 
Ionbsy (aussi (CWA Cleeteys SEG) | erin sn eee sted nee. 60:8 Q@ (all 
Ane Oy) — Ie Or te aly in ete ieee rer terse early) 
Q 641 dead 4-17-15 Total gas... antec 26 o7: 56 2 
9 647 (younger) 
1913 | (18) jist ( 6)= 7.246 + 6 (2) 1.482 + 11 1:6 9 (all 
2nd ( 5)= 8.0624 2 (2) 1.585 + 3% late) 
1914 | (51) |Ist (25)= 7-478 + 9 (5) 1.653 + 70 17 #219 9 
2nd (24)= 8.403 + 4 (4) 1.793 + 59 
1915 | (45) Ist (22)= 7.624 + 11 (1) 1.715 + 166 It. 321 Qos) 
2nd (22)= 8.481 + 3 (1) 1.970 + 171 
Q 647 dead 2-16-16 Totales na. sospectet 29 of: 46 9 


« The entire egg loses weight on standing; the yolk gains weight on standing. 


egg of the pair. There were occasional reversals of this relation 
and also occasional pairs with quite nearly equivalent weight. 
In the eggs produced by hybrids this relation did not obtain at 
all. Illustrations of these differences in weight between the egg- 
yolks of first and second egg of the clutch may be seen in any of 


RIDDLE: CONTROL OF SEX RATIO 301 


CAA“ T 7: 
SEX CONTROL AND ANOWNM CORRELATIONS 1 PIGEONS . 


SPING 


NO / SHOWS COSTAIRITNE SIZE OF EGGS OF ALZGA A) 
AND OPIENTALIS (C(O). 


332 RIDDLE: CONTROL OF SEX RATIO 


TABLE 7 


Summary oF PARALLEL BREEDING AND CHEMICAL STUDIES ON THE EaGGs oF 9 
T. orientalis No. 500 X St. alba No. 410—ror THE YuHaAR 1912 


ALC. RESULT 

mom AN'L'S OR |W. OF! cory- 

ase YOUR | BLE miner eae Ext. Ash | H.0 Energy total 
4-13 Broken when found 
4-15 Broken when found 
5-26 | 159 2.330) 72.65) 18.32] 25.44) 5.28 | 4.85 | 57.01 7405 
5-28 160 2.660} 72.45) 17.54) 25.63) 5.25 | 2.62 | 54.82 8990 
6- 7 Inc. Only one egg laid.|......]......]....-- Dark ¢& 
6-15 Tesi ota) ook see Stated oe eos | eee | eres Dark o& 
6-17 Inc. ‘Very lareelege a areal alee White 9? 
6-24 aCe [ean ee nee Mon Waal ney ibaa et llic og ot No dev. 
6-26 10 Coe mane | ae Deter er Ves steyes| I Ceseate| lets eerata Pc aise Dark 


7-3 186 2.026] 71.95] 16.49) 26.00} 3.63 | 2.43 | 56.05 6714 
7-5 187 2.330) 72.27) 19.18) 26.55) 3.75 | 1.93 | 55.22 7881 


7-15 16 cy oe eet ne Meee Maareenar brn al ee oe ot lease Dark o 
7-17 Trice fice laste. eae] othe ets leer cl eenee Dark @ 
7-23 192 2.422) 72.42) 17.82] 25.88] 3.82 | 1.80 | 55.84 8061 

7-25 193 2.720) 72.45] 18.88] 25.96) 3.86 | 1.81 | 55.33 9296 

8- 2 J Grek ||) | Ie eee | era | scart | byete cet aay ciaicieal levcoays,> Dark o 
8- 4 1 sCometil| Sal | earns nearer pane (eer rare feb tees ta] Ir eestas .| Dark &# 
8-13 Arye Na | ata oes] ee Sasa Be, opel ameatet tel | etre eel ae, eee No. dev. 
8-15 iliac Vere Bnes let ceae |e IMac. tes Gor oec'| ceva es eee Dark 
8-23 Voters. Siig) MLL.) Ria ea Rg ote) Neer care [eas Ao Lett ora No dev. 
8-25 iT 3 | Sicha | Sepee y |PTs chs stal| ake he eects runic White 92 
9-15 A aycryenm lt or che file coke eeamareard | cio taco | erty oll aren Aha eet White 2 
9-17 VG: 2 Wetec Wal eee See | osetia | gdeetesil ees yeceell Pacbeeertell eee White 2 
11-29 259 2.700] 73.17) 21.40)425.23)......)...... 55.52 9323 

12- 1 260 2-715\)-73..02)' 21 .63}225. 38)... .\..--.- 55.39 9383 


2 Calculated. 


RIDDLE: CONTROL OF SEX RATIO 333 


the appended tables in which yolk weights are given (Tables 4, 
5, 6, 7, 8). 

Other facts concerning the yolk weights which soon came to 
light were, that the yolks from an individual bird become larger 
in the autumn, particularly if the bird is made to lay numerous 
eggs (i.e., overworked) during the season. A schematic repre- 
sentation of the dimorphism of the ova, and of their increase in 
size from spring to autumn is shown (under 1) in Chart 1. A 
further fact of kindred nature was learned when the study was 
extended over a period of years, namely that the egg-yolks of 
an individual bird tend to become larger as the bird grows older; 
the yolks of the spring, however, are usually smaller than those 
of the previous autumn, though larger than those of the previous 
spring (Table 6). These facts are now established by accurate 
weighings of more than 12,000 yolks, freed and separated from 
their surrounding shell and albumen. 

The details of the chemical analyses of one series of eggs ob- 
tained in 1912 are given in Table 7. These details we need not 
here consider, but it will be observed that we find larger amounts 
of the various chemical fractions (excepting water) in the fe- 
male-producing egg than in the male-producing egg. This 
holds true alike for the female-producing egg of the clutch, and 
for the late eggs, which under these conditions are predomi- 
nantly female-producing, as compared with the group of earlier 
eggs which under the conditions of the generic cross are rela- 
tively male-producing.* Not only does the size of the egg in- 
crease with its later position in the series, i.e., with lateness of 
season, as shown by a mere comparison of the yolk weights of 
such a series of eggs, but the percentage of energy-yielding or 
stored materials increases as much, or probably more, than is 
indicated by the size, or net weight of the yolk. The per cent- 
age of water, we shall later see, is greater in the male-producing 
eggs. . 

For our present purpose the importance of the results of these 
and other analyses is that they conclusively show: (1) that the 

§ In this particular series, 8 of the first 9 eggs incubated produced males; the 


egg of this group that hatched a female was “‘a very large egg.’’ The last three 
hatches were females, : 


3384 RIDDLE: CONTROL OF SEX RATIO 


male-producing egg of-the spring is an egg that stores less ma- 
terial than does the female-producing egg of the autumn. (2) 


TABLE 8 


StorEep Enercy or Haes (1914) or Streptopelia risoria (9 558) As DETERMINED 
BY THE Boms CALORIMETER 


NO. DATE WT. OF YOLK ENERGY PER CENT DIFF. 
665 Al 6-6 21.010 23,358 
666 A2 6-8 0.970 © 3,175 b_ 5.8 
674 Bl 6-19 0.855 2,807 
675 B2 6-21 1.000 3,245 +15.6 
699 * Cl 7-14 1.145 3,815 ? 
700 C2 7-16 1.463 5,008 +31.3 ? 
728 D 8-30 1.395 4,812 

E 9- 9 or | 10 soft shell, bro|ken. 

F1 10-17 soft shell, brojken 

F2 10-19 soft shell, broj|ken 
770 G1 11- 6 1.440 4,837 (?) . . 
771 G2 11-8 1.720 5,797 +19.8 ? 
774 H1 11-20 1.590 + sl. loss 4,906 + 
775 H2 11-22 1.780 6,015 +22.6 ? 
776 Tl 12- 1 1.640 5,614 
777 I2 12- 3 1.820 6,255 +11.4 
781 J1 12-12 1.585 5,302 
782 J2 12-14 ' 1.690 5,601 + 5.6 
791 K1 12-23 1.485 5,266 (?) 
792 K2 12-25 1.718 5,880 +11.7 ? 


@ This egg was not only the first laid during season, but first during life of 
this bird. a 


> The percentage differences are based upon a value of 100 per cent for the 
smaller egg of the pair. 


That the male-producing egg of the clutch stores less material 
than does its female-producing mate. (8) That the eggs of old 


RIDDLE: CONTROL OF SEX RATIO 335 


females store more materials, and—as has been noted—yield a 
higher percentage of females, than do birds not old.? There- 
fore, it is.evident that the egg of female-producing tendency is 
one whose storage metabolism is high, as compared with eggs. of 
male-producing tendency. Moreover, the analyses show that 
during the season successive clutches present higher and higher 
storage, i.e., the earlier clutches store less—are more male-like; 
the later ones all store more—are more female-like—and _as.al- 
‘ready noted the eggs of the low storage period give rise (in the 
generic cross) to males, and those of the high storage period 
produce females. 

We here obtain a close view of that upon which sex, difference 
rests. And the facts are now quite beyond question. Un- 
mistakably, less storage and high storage pertain respectively to 
the male- and female-producing germs. Unmistakably, our pro- 
cedures, connected with generic cross, season, and overwork, 
delivers males from the smaller storages in the earlier eggs. Un- 
mistakably, the procedures raise the storage in all of the later 
eggs, and unfailingly we then find that these eggs yield only, or 
predominantly females. And if we eliminate the factor of wide 
(generic) cross and mate the female with one of her own or a 
very closely related species (Table 5), then we see that the pro- 
duction of males and females coincides from the first with two 
storage values—with two sizes of eggs (yolks) in the clutech— 
males from the smaller first, females from the larger second. 
Only after overwork and season have raised the storage value 
of the eggs is this situation seriously disturbed. And the dis- 
turbance—associated with an increase in the storage metabolism 
of all the eggs—delivers as before, an excess of female offspring 
(Tables 4, 5, 6). 

The progressive increase in storage capacity of the eggs during 
the season—under overwork—is to be interpreted as a decrease 
in the oxidizing capacity of these same eggs. Living cells in 
general dispose of ingested food material by storing it or by 
burning it. If oxidized the products of the oxidation are re- 
movable and do not serve to increase the bulk of the cell. The 


7See Tables 4, 5, 6. 


336 RIDDLE: CONTROL OF SEX RATIO 


low-storage capacity of the male-producing eggs as compared 
with the high storage capacity of female producing eggs is there- 
fore an index of higher oxidizing capacity or as more usually 
stated, a higher metabolism of the male-producing eggs as com- 
pared with the female-producing eggs. 

We may next examine the percentages of water in the eggs 
of spring and autumn, and in the two eggs of the clutch. These 
figures for one series of analyses are given along with other 
analytical results in Table 7. They show a higher water con- 
tent for the eggs of the spring (male-producers). Indeed, each 
pair of eggs from the first of the season onward has a slightly 
higher moisture value than the pair that follows it. The analyses 
further show a higher percentage of water in the first egg of 
the clutch, ie., in the male-producer, than in the second or 
female-producer in all cases. 

If the results of my nearly 900 analyses all ran as smoothly 
as do the 8 of this series there would be no doubt of a perfect 
correlation of high moisture values with small eggs, i.e., with 
male-producing eggs—both small eggs of season, and small eggs 
of individual clutches. The results throughout, however, are 
not so uniform and smooth as here; there are some series which 
seem seriously to depart from the order noted above. These 
cannot be adequately discussed here. We can, however, record 
our own belief that the situation represented in the table is, in 
the main, indicated by the moisture determinations obtained 
in the analyses of eggs produced by pure species. Two ad- 
ditional methods of determining the amount of water in the 
yolks, give a satisfactory confirmation of the conclusion that 
the male-producing ovum contains a higher percentage of water 
than does the female-producing ovum. 

It may be remarked at once that the two facts—a higher 
metabolism, and a higher water value in the same egg (the 
male-producing one)—are not to be regarded as a mere coinci- 
dence. They are related facts, essentially correlated in that 
the more hydrated state of these colloids, which contain only 
54 to 59 per cent water, is certainly a more favorable state for 
a higher rate of (oxidizing) metabolism than is the less hydrated 


RIDDLE: CONTROL OF SEX RATIO 337 


state which better corresponds to a condition favorable to in- 
creased storage.’ 

The results of these analyses (as well as the calorimetric deter- 
minations to be mentioned later) have an important relation 
to the question of a modified or differential maturation, by 
which the changed ratios might be explained. Bearing on this 
point we may here make the following observations: It has 
been seen that the sex actually realized corresponds in fact to 
levels or grades of metabolism; and we now note that the (stor- 
age) metabolism which was measured was complete before the 
beginning of maturation, so that if such a differential maturation 
should occur it must be looked upon not as a cause but rather 
as a result of the establishment of that grade of metabolism which 
does here, and under all of the several known conditions, in the 
clearest way accompany and correlate with each particular 
sex. 

But, any assumption of a differential maturation, even'as a 
result of or response to these impressed levels of metabolism, 
brings with it more difficulites than it clears up. Among these 
it brings the paradox of a rigid selection in favor of the male- 
producing chromosome-complex in the maturations of the 
spring, and an equally rigid selection against this same complex 
in the autumn. Again, it is easily shown by simple breeding 
tests that such differential maturation does not occur in the 
spring at least when the female is mated to her own or a closely 
related species; so that a further assumption would have to be 
made to the effect that it is the prospective fertilization by a 
sperm from a wider cross that determines the course of matura- 
tion! Furthermore, our data on the sex-behavior of series of 
females from such a wide (generic) cross show that if the male- 
producing complex was indeed eliminated from the eggs that 
gave rise to one-half of these females (produced under overwork) 
these same chromosomes cannot be the real or sole cause of 


8 For example, Overton found that withdrawal of water from the cells of 
Spirogyra was followed by an increased storage or accumulation of starch, ete. 
Embryonic tissues generally have high water content and show most rapid di- 
vision, differentiation and growth (not storage), etc. 


338 RIDDLE: CONTROL OF SEX RATIO 


masculinity, for as we shall see later a part of these females are 
strongly masculine, and indeed they show various grades of 
masculinity. The evidence against a differential maturation as 
a basis for an interpretation of the controlled sex ratios of 
pigeons is so strong as to cause its rejection, even if the essential 
constructive facts on the nature and basis of sex had not yet 
been learned. 

The storage metabolism of many male- and female-produc- 
ing ova, both in reference to egg of clutch and to position in 
the season, has been determined by means of the bomb calori- 
meter. The method is very accurate and the results are entirely 
convincing. The stored energy, or heat of combustion, of 
nearly 400 egg-yolks has been determined. One such series of 
determinations, (made in 1914) in which all available eggs of 
a particular female were burned is shown on Table 8. It will 
there be seen that the first clutch or pair of the season bore a 
higher caloric value than the second pair, but is otherwise the 
smallest of the year. Beginning with the second clutch laid 
in June the succeeding clutches to-December 1 bear higher and 
higher heat values. In all clutches too, except the very first,°® 
the second eggs show a higher storage of heat units than do the 
first of the clutch. Here we find the conclusions reached from 
studies on the wieghts of yolks, and on yolk analyses, fully 
confirmed by a method in which the error involved in the de- 
termination is wholly negligible. The most accurate method 
for the study of the storage metabolism of male and female 
producing ova give too the results most consistent with the 
breeding data. In other words, we could say, if we wished to 
make merry with our colleagues, the cytologists, that we here 
get closest to the facts of sex when we burn our chromosomes! 

The energy values obtained from the burned yolks, permit 
an indirect comparison of the water values of the male- and 
female-producing eggs of the clutch. Such a comparison in- 
dicates, as in the chemical analyses, a higher percentage of 

* Professor Whitman has observed that the very first egg in life or of the 


season is more likely to throw a female than is the first of the clutch of the imme- 
diately succeeding clutches. 


RIDDLE: CONTROL OF SEX RATIO 339 


water in the male-producing ovum. In addition to these two. 
methods of studying the water values of the two kinds of eggs 
the value has been obtained direct, by desiccation, on a con- 
siderable number of samples. The three methods confirm each 
other. A little later we shall make a further application of the 
observed facts of higher water values, and of a higher metabo- 
lism in the male-producing ova. 

Let us now very briefly consider the other kinds of obser- 
vations that have been made on the series of eggs, from spring . 
to autumn, produced under crowded reproduction by generic 
crosses, as these are schematically represented in Chart 1. 
Curves 2 and 3 on that chart represent facts which were first 
observed by Professor Whitman on these series of eggs. The 
curve entitled ‘‘developmental energy’ (No. 2) represents the 
observed fact that more of the eggs of spring show the capacity 
to develop than do those of autumn; and by the use of a con- 
tinuous (not broken) line or curve is indicated the further fact 
that the first eggs of the clutch bear throughout the season a 
similar relation (of higher fertility) to the second eggs of the 
clutch. The curve marked 3 and designated “length of life” 
tells again of an advantage possessed by the earlier hatched 
birds, and of a more limited life-term affixed to the hatches 
from the later ‘‘overworked”’ eggs. It is probable, moreover, 
that within the group of clutches giving rise to females only, a 
longer life-term falls to those birds arising from the first egg of 
the clutch than from those arising from the second of the clutch. 
Here, then, as in the preceding curve (2), the smaller eggs of 
both clutch and season are the eggs which give in their develop- 
ment the tests of ‘‘strength and vigor,’ while the larger eggs 
of clutch and season more often display ‘‘weakness.”’ 

The data which justify curves 4 and 5 as represented on the 
chart have already been considered. Of the observations upon 
which curve 6 is based we shall here say only that in general 
the weight of the parent bird is greatest at the season when the 
weights of the yolks being produced are smallest, and that when 
the largest yolks of autumn are being produced the weight of 
the parent bird is the smallest of the year. Tables 4 and 5 


340 RIDDLE: CONTROL OF SEX RATIO 


were prepared originally to make clear certain observations on 
size of off-spring in relation to their origin from eggs produced 
under overwork, after continued overwork, and in relation to 
the order of the eggs in the clutch. The tables themselves 
tell much of their story and we here forego a further considera- 
tion of them (see Rippie, 716, p. 406). 

The seventh curve of Chart 1 refers to a long and rather 
large series of tests of the sex-behavior of series of birds such 
as those whose origin is indicated in Table 3 (series of 1908). 
We have here an opportunity to study and. compare sex phe- 
nomena of particular birds whose sex we have reason to believe 
had been reversed from its initial sex-tendency; that is to say, 
where successive pairs of females have originated from succes-’ 
ive pairs of eggs in the autumn, under overwork, we have the 
reasons already given for believing that some or most of such 
females arising from first eggs of the clutch have had their 
metabolism depressed to a point sufficient to make them fe- 
males; but the second eggs of the same clutches should by the 
same means have been carried to a still more ‘‘feminine”’ level; 
and though both are females, it seemed possible to differentiate 
the one sort from the other, and this has been successfully done 
in a series of tests which now extend through a period of nearly 
five years. Each female has been given about nine tests, each 
of six months duration, with (for the most part) another female. 

In this study, then, female is mated with female and male 
with male. Such pairs, from a very few selected pairs of par- 
ents, are kept mated for a period of six months. Most of the 
birds used, for lack of success with the incessantly fighting 
males, have been females, and most of the nine or ten successive 
tests with each bird have been made with her own sisters. The 
members of the pair are kept apart except when under obser- 
vation; when put together, as is done twice daily, the records 
are taken of those females of the pair which behave as males 
in copulating with their mates.. Three facts are definitely es- 
tablished by the data obtained: (1) The females of the orient- 
alis X alba cross (they are dark in color) are more male-like in 
their sex behavior than the females of the reciprocal cross (these 


RIDDLE: CONTROL OF SEX RATIO 341 


are white in color). (2) Females hatched from eggs laid earlier 
in the season are more masculine in their sex behavior than are 
their own full sisters hatched later in the season. And, several 
grades of females can be.thus seriated according to season of hatch- 
ing. (8) The female hatched from the first egg of the clutch is 
more masculine than her sister hatched from the second of the 
clutch in a great majority of the cases. And in nearly all these 
latter mat‘ngs the more masculine bird is so predominantly 
mascul ne that she takes the part of the male a full 100 per cent 
of the time in copulating with her very feminine clutch-mate 
sister. (See Rippin, ’14a). 

I may remark in passing that the effect of testicular and ovar- 
ian extracts (suspensions) have been studied in connection with 
the work on sex-behavior. The results have clearly shown 
that the sex behavior of a pair of females is modified by the 
intra-peritoneal injection of testis (pigeon) extract into the one 
and ovarian (pigeon) extract into the other. In one case, for 
example, the more ‘“‘feminine”’ female of a pair was given testis 
extract and her more ‘“‘masculine”’ mate received ovarian ex- 
tract. After the injections the bird formerly more feminine, 
16 copulations as a male to 23 by her consort, became very 
much the more masculine, 27 copulations as a male to only 2 
by her consort. 

To one other kind of fact concern ng the effects of reproduc- 
t ve over-work ‘n changing the developmental and sex phen- 
omena of the germs of the later part of the season, we ask a 
moment’s consideration. 

It has been found that some females dead at relatively ad- 
vanced ages show persistent right ovaries. The right ovary 
in pigeons normally begins degeneration at or before hatching 
and is usually who'ly absent from the week-old squab. In 
our study it soon became evident that the persistent right ovar- 
ies were found almost exclusively in birds hatched from eggs 
of overworked series. Further study has shown in addition 
that they arise almost wholly from the eggs of autumn, and 
predominantly then from -the second eggs of the clutch—that 
is from eggs otherwise known to have greatest or strongest 


342 ‘RIDDLE: CONTROL OF SEX RATIO . 


female-producing tendency. These ovaries have sometimes 
weighed half or more than half as much as the adult left ovary 
with which they were associated, and have been found in such 
birds dead at all periods from a few days to twenty-four months. 
We here attempt no adequate description of this situation, but 
one can not have observed the frequency of the persistence of 
this ovary in the birds hatched from the eggs otherwise known 
to be the most feminine from these overworked series, without 
conviction that the same pressure which carries the eggs of 
spring from male-producing to female-producing levels, also 
carries the earlier female-producing level to another yet more 
feminine. 

The several kinds of facts just reviewed in connection with 
Chart 1 afford clear evidence that sex and characteristics other 
than sex such as fertility and developmental energy not only 
bear initial relations to the order of the egg in the clutch, but 
that sex and these other characteristics are progressively modified 
under stress of reproductive overwork, until at the extreme end of 
the season certain aspects of femininity are abnormally or un- 
usually accentuated. In the light of these facts sex reveals 
itself as a quantitative modifiable character. And an associa- 
tion of modifiable metabolic levels with the flux and change of 
sex, or of sex ratios, has been found and described in precisely 
this same connection. 

Let us now take these facts with us in a rapid survey of some 
experimentally induced and puzzling sex-ratios, and also into 
a brief consideration of some important facts of sex that have 
been learned from embryonic and post-natal stages of organisms. 

The evidence that higher water values and higher metabolism 
are associated with male-producing eggs, lower water values 
with female-producing eggs, is of first importance in connection 
with our own generalization as to the germinal basis of sex- 
difference; and is further of much interest as being the means 
of demonstrating that in the—as I believe—several valid cases 
of sex-control now known, one thing in common has really been 
effected; this, though the work has.been carried out on a con- 
siderable variety of animals and though the procedures have 


RIDDLE: CONTROL OF SEX RATIO 343 


themselves been most various. The thing that seems to have 
been effected in all cases has been the raising or lowering of the 
general metabolism of the treated germs. In probably none of 
the cases in which these experimentally induced abnormal sex- 
‘ratios were obtained—in other animals than the pigeon—has the 
observer been able definitely to eliminate all the possibilities of 
the continued determination of sex by the sex-chromosome; but 
several observers have been able to eliminate one or more of 
these possibilities for their material. And all of those experi- 


TABLE 9 


Time oF FERTILIZATION AND THE Sex Ratio In CATTLE 


AUTHOR TIME o': 9 AUTHOR TIME oi: Q 

Thury and Early 0:7 Early 31:51 

Cornaz......... { Late 22:0 Braseellbe br a Late 42:34 
ane Early 8: 10 

SUS Sea { eee (| Early 123: 125 

aries. Middle 67: 58 

: Early 3: 10 Sas: Late 65: 42 
Dusinge oe. s.-2 Latent: 


Early 134: 178, ratio = 75.3 @:100 9 
Total® {Middle 67: 58, ratio = 115.5 1: 100 9 
Late 77: 44, ratio = 175.0 &: 100 9 


@ Work cited by Diising. 
+ Omitting the data submitted by Cornaz in the first announcement of the 
theory. , 


ments which strongly suggest a real sex reversal or control can 
now be shown to be in alignment with one or more of the basic 
facts of sex control now known in the doves and pigeons. When 
the conditions of these experiments have been such as to lead 
us to expect an increase of the metabolism, males have been pro- 
duced in excess, and when the conditions imposed have been 
obviously capable of depressing the metabolism of the treated 
germs, these have yielded an excess of females. These facts, 
therefore, afford much reason for the opinion that sex has been 
controlled or reversed in a number of very different animals. 


344 RIDDLE: CONTROL OF SEX RATIO 


The observed relation of the time of fertilization to modified 
sex-ratios in cattle is summarized in Table 9. Thury reported 
in 1862 that from fertilizations made in the early period of heat 
in cattle an excess of females were produced; and that later 
(delayed) fertilizations give rise to an excess (all according to * 
Thury) of males. Similar experiments have been several times 
repeated and these repetitions have all shown an excess of one 
or the other sex in accordance with such early or late fertiliza- 
tion.!° The facts as reported by the several observers, and the 
totals, are given in the table. We postpone for a moment a 
discussion of the situation presented by these data except to 


TABLE 10 
TIME OF FERTILIZATION AND SEX RaTIO IN SHEEP 
{| Matings in October, 1899 co 10:2 26 = 72.0 per cent Q © 
Matings time unknown, 
Bell¢....... [SOO Mane ee ar Aen ae o& 179: 2 166 = 48.0 per cent 9 
Matings after Novem- 
berpl5 a] SOOM eeraeee ot PRS SO 8 = 11.5 per cent 2 


@ Records of a neighboring flock supplied to Dr. Bell by Mr. Macrae. 


draw attention to the probability that in late (delayed) fertiliza- 
tion the ovum takes up water before fertilization and gives an 
excess of males. 

Connected with these facts obtained from cattle are some par- 
tially similar data for sheep. From records obtained by Dr. 
Alexander Graham Bell (14), made primarily with the object of 
learning whether certain conditions have an influence on “‘twin- 
ning” in sheep, the materials for Table 10 have been taken. 
Here, again, as in cattle there is probably some evidence for an 
increased male production from delayed fertilizations. 

Experiments on the frog and the toad have afforded evidence 
for the control of sex. Richard Hertwig (’06, 712), and later 
Kuschekewitch (710), allowed frog’s eggs before fertilization to 
“overripen,”’ a process during which the eggs take up water— 


10 The use of the terms early and late fertilizations assume that some ovula- 
tion occurs either immediately before, or shortly after, the beginning of heat, 


RIDDLE: CONTROL OF SEX RATIO _ 345 


and obtained (the latter author) in some cases a total of 100 per 
cent males (Table 11). Dr. King (12) did the converse of this 
experiment with toad’s eggs—withdrawing water from them be- 
fore fertilization—and obtained nearly or quite 80 per cent of 
females in cases where the mortality was less than 7 per cent. 

The evidence afforded by these experiments on the frog and 
the toad is thought by many to be inconclusive as evidence for 
real sex control. Though selective fertilization has been elimi- 
nated as a possibility by Kuschekewitch, we do not know which 
is the heterogametic sex in amphibia and there also remains the 


TABLE 11 


EXPERIMENTALLY MoptiFrigep Sex. Ratios IN Frocs AND Toaps 


: RESULT 
AUTHOR TREATMENT fofd 
fou 9 
per eent 
271 0 100.00 
Hertwig, R........] o) | Delayed fertil. (+ HO) 
= ; 88.88 
Kuschekewitch .... Delayed fertil. (+ HO) 299 0 100.00 
No delay + HO 62 4] 60.20 
cs) 
S 106 275 27 .66 
1Gnotapel alg Dp eeaacee ~ | No delay — H20 
85 289 22.73 


possibility of parthenogenetic development to account for the 
excessive male-production in the experiments with the frog. 
But this appeal makes it impossible to explain the great excess 
of females obtained by Dr. King on the eggs of the toad, where 
a selective mortality is definitely excluded, and leaves such 
doubters to lean upon the rather discredited staff of selective 
fertilization—a proposition wholly disproved for the related frog 
and for the pigeon. It may be noted, however, that on the basis 
of our present knowledge of the ‘‘sex-differentials’’ (to be con- 
sidered later) in the pigeon’s eggs both of these experiments 


346 RIDDLE: CONTROL OF SEX RATIO 
might have been predicted to result as these three investigators 
have reported. 

The modified sex ratios obtained from the four types of ani- 
mals just mentioned were all obtained through action upon the 
eggs, or the egg-stage of the organisms. Some important ex- 
perimental work, and other very significant physiological and 
chemical study, has been done on sex in the embryonic and adult 


CHART 2 


Bonellia; Free-martin; Inachus; Frog; Pigeon; Duck; Fowl; Pheasant; Sheep; 
Human; Stag. 


( *high per cent H2O (?) f {  { (blood) low per cent 
Cow 9 low per cent (H2O (?) rofl fat 
| lees METABOLISM 
Human 
low fat and P. (blood) high per cent 
of [Bio METABOLISM Q fat 
high per cent H20 Low Merraso.uism 
Egg Pigeon { >Adult 
fe fat and P. Row! & (blood) low fat and P. 
@ , Low Merazorism @ (blood) high fat and P. 
low per cent H2,O 
Frog o@ high per cent H2,O Crab o& (blood) low per cent fat 
Toad 9 low per cent H2,O Q (blood) high per cent fat 
Fdatines ee from change of food and increased oxygen supply. 
Q’s from unchanged food and lesser oxygen supply. 
Daphnids { sex-intermediates—sexual or asexual reproduction influenced by 
conditions. 3 
Moths sex-intermediates—quantitative germinal basis. 


stages of the organism. Something can here be gained by 
grouping and treating these several results in a single diagram. 

Now that the basic problem of sex has been shown to be essen- 
tially a question of metabolism, a department of physiology and 
biochemistry, we shall be able to note in connection with Chart 
2 (where the principal known facts concerning the relation of 
metabolism to sex are diagrammatically arranged) that a num- 


RIDDLE: CONTROL OF SEX RATIO 347 


ber of data bearng on adult sexual differance of the sort we 
most require are already at hand. 

Turning now to the diagram we note that egg and adult stages 
are first distinguished. In the egg of the pigeon we have iden- 
tified maleness and femaleness by three differentials. Female- 
ness in the egg-stage being accompanied by low metabolism, 
lower percentage of water, and higher total fat and phosphorus, 
or of phosphatides. Maleness is here accompanied by high 
metabolism, higher percentage of water, and lower total fat and 
phosphatides. Now there are valid reasons for treating these 
three differentials not as separate and disconnected facts, but 
’ rather as aspects or corollaries of the same fact. For example, a 


TABLE 12 


Sexvat DirrerEences oF Fat AND PHOSPHORUS IN THE Boop or ADULT FowLs 
AND oF Man 


AVERAGE RELATIVE 


ox monsts waethlp geet op NCU Oe 
Males (roosters)...........0....0555 Ree 15.45 6.43 100 
INonelawing females... cc .c.c ee ee eee eens 17.87 7.42 115 
Hnytaperemalest. cA oes lets wed eek eee seek 27.80 13.15 205 
UVM en (TANI opstetate. sis). cic isy suave eieiataye tienen cae 141.4 
er ale gn (OMEN ey «chet delet occ elenten oe. 226.0 


high metabolism in a cell is consonant with less storage of fat and 
phosphatides, and with a more highly hydrated state of the cell- 
colloids. It follows that where data for either of these three 
differentials are at hand, for either the germ or adult of any ani- 
mal, we have in such data evidence of the kind we’are looking 
for, i.e., evidence for the association of a given type of metab- 
olism with the germ or adult of a given sex. 

For what forms then are such data available? . And, what is 
now known of the persistence of this definite type of differentia- 
tion of the two kinds of sex germs into adult stages of the two 
sexes? Recently, in my laboratory in codperation with Mr. 
Lawrence (’16), it has been shown that one of these differentials 


348 RIDDLE: CONTROL OF SEX RATIO 


—or one aspect of the differential which our own work has dem- 
onstrated in the egg—is clearly continued in the blood of the adult 
male and female. Fowls were substituted for doves in this 
case in order to increase the size of the samples and thus increase 
the accuracy of the analytical results. The blood of the male 
contains less fat and less phosphorus—just as the male-producing 
egg contains less of these same elements. The data further 
show that the sexually active (or actively functional) females 
depart most widely from the male, while sexually inactive females 
occupy an intermediate position in respect of the amounts of 
these constituents found in the blood (see Table 12). 

The results afford fairly clear evidence that in birds the meta- 
bolic differences of male and female germs persist in the male and 
female adults. 

In mammals too these aspects of sexual differences of the 
adults have been fully demonstrated. Almost simultaneously 
with the above determinations on birds, data were published by 
Goettler and Baker (’16) which (as we have pointed out, 16) 
show that the blood of the human male contains less fat, that 
of the female more. Further, the basal metabolism of the human 
male and female has recently been accurately determined by 
Benedict and Hmmes (715); they find that the metabolism of man 
is 5 to 6 per cent higher than that of woman. 

Have we any measure of either of our differentials in any 
mammalian egg? I think that the experiments on sex-de- 
termination in cattle, together with an observation by van der 
Stricht, afford some evidence that the water content of the male- 
producing egg is high, and that of the female-producing egg is 
low. No one definitely knows whether the ovum of the cow 
absorbs water in the Fallopian tubes in this interval between 
ovulation and fertilization, but we do know that every amphib- 
ian, reptilian, and avian egg that has been investigated does 
absorb very appreciable amounts of water while being passed 
from the ovary to the exterior. And van der Stricht has de- 
scribed phenomena of growth or swelling of the yolk-granules of 
one mammal—the bat—which, I am sure from my own studies 
on yolk, indicate the taking up of water by the egg of this mam- 


RIDDLE: CONTROL OF SEX RATIO 349 


mal. It is highly probable, therefore, that precisely that time 
relation which leads to an excess of males in cattle is preceded 
or accompanied by an increased hydration of the ovum. In 
mammals therefore there is some evidence that a shift of the 
metabolic level—as indicated by one partly known differential 
—is associated with the observed changes in the sex-ratio of the 
germs which are thus modified. Further, in the adult of one 
mammal—man—two of the three sex-differentials have been 
definitely demonstrated. These results for both the egg and 
adult stages of the mammal are at every point in complete agree- 
ment with our data for both the egg and adult stages of the 
bird. 

How now do the controlled sex ratios obtained in the frogs and 
toads appear in the light of the sex differentials of our diagram? 
Clearly the data given in Table 11 arrange themselves in per- 
fect agreement with the metabolic differentials which obtain in 
birds and mammals. The data of that table eliminate ‘‘de- 
layed fertilization” as such as being a factor and show that the 
altered sex ratios correspond with increase or decrease of water 
as the sole known differential. 

We next give a moment’s consideration to an adult stage in 
which a change in metabolism was observed in connection with 
sexual changes. In the spider-crabs Geoffrey Smith (11) 
showed that both the blood and the liver of the adult male crabs 
contain less fat than do the blood and liver of the females. 
Here once more the facts concerning one of the sex-differentials 
is known and is in complete accord with all the preceding cases. 
In these spider-crabs, known to be sometimes castrated by para- 
sites, Smith and Robson were able to show, moreover, that the 
parasitized male crabs, which under these conditions gradually 
assume several female morphological characteristics, are also found 
to have assumed the type of fat metabolism which characterizes the 
normal female crab. How much these facts contribute to, and 
how completely they adjust themselves to, our own general 
theory, will be realized only after a moment’s reflection. Re- 
cently Kornhauser (’16) has found some of these conditions also 
in Thelia. 


350 RIDDLE: CONTROL OF SEX RATIO 


A glance at the diagram indicates three other groups of ani- 
mals which experimental work has thrown into the general ques- 
tion of the control of sex. The information at hand for these 
forms does not so expressly concern the egg as does that from 
the preceding cases, but all of these latter groups are concerned 
with early stages—some of them with the generation preceding 
the egg whose sex seems influenced by conditions. The results 
of studies of the first of these groups—Hydatina—are of such a 
kind as to show that they are in general accord with the meta- 
bolic differentials of all of the previously mentioned cases of sex- 
control. One can scarcely doubt that change of food and in- 
creased oxygen supply are consonant with increased metabolism, 
just as the studies of Whitney (’14 and later) particularly, and 
later of Shull (16), have shown that these changes lead to the 
production of male-producing daughters. 

The second of these groups—the Daphnids—have been stud- 
ied by three independent investigators who agree upon two 
points that are of importance in the question of the control of 
sex, and to the general theory of sex as stated here, though the 
results throw little light on precisely what is causally involved. 
Issakowitch (05), Woltereck (11), and Banta (’15) all find 
numerous sex-intergrades in a material in which all agree that 
the type of reproduction—sexual or asexual—is influenced by 
environmental conditions. All further agree that ‘unfavorable 
conditions” (or is it a change from favorable conditions?) tends 
toward sexual reproduction, while ‘favorable conditions” favor 
asexual reproduction. ; 

In the third of these groups—the moths—the studies of Gold- 
schmidt (712, ’°14), Goldschmidt and Poppelbaum (’14), Harri- 
son and Doneaster (’14), and the work of Machida, have dem- 
strated again sex-intermediates of various grades. Moreover, 
it has been shown that from among the various geographical 
races of moths certain matings can be arranged which produce 
rather definite types of male- or female-intermediates—or sex- 
intergrades as Goldschmidt elects to call them. And further, from 
pairs involving still other species still other levels or grades of 
sex-intermediates may be-freely obtained. A more or less fac- 


RIDDLE: CONTROL OF SEX RATIO 351 


torial basis of the phenomena has hitherto been used in the 
discussion of these results; but recently Goldschmidt (’16) has 
stated that ‘‘very important new facts will be published later 
which will probably enable us to replace the symbolistic Men- 
delian language, used here, by more definite physico-chemical 
conceptions.”’ Such newer descriptions—we would say—is 
wholly in line with the requirements of present data on sex. 
In Whitman’s and our own material it has been clear from the 
first that the results far overstep the possibility of treating 
them in Mendelian terms, for it has been apparent from the 
beginning that we have had to do not with three or four points 
merely, but with a flowing graduated line. In the work with the 
moths, however, sex is clearly described in quantitative terms, 
and it seems fairly certain that when the functional basis of sex 
shall have been identified it will be found that sex accords 
with metabolic grades there, as it does elsewhere. 

It is clear then that all of the animal-forms for which there is - 
reasonable evidence of sex-control show important correspon- 
dences with the situation fully elucidated in the pigeons. And 
that where the sex-differentials known to exist in the pigeon’s 
ova have been traced in adults of the two sexes, the parallel 
rigorously holds there also. A general classification of male and 
female adult animals on the basis of a higher metabolism for the 
one and a lower for the other, was indeed made by Geddes and 
Thomson (’90) many years ago. It now seems beyond ques- 
tion that this conclusion of these authors is a correct and impor- 
tant one. 

It remains to point out that another very old and much 
worked line of investigation supplies further confirmatory evi- 
dence for our present point of view. Studies on the effects of 
castration, gonad-transplantation, and gonad-extract injection, 
constitute a large body of observations which deal with sexual 
phenomena associated with the internal secretions of the sex- 
glands. These internal secretions, let it be remembered, are 
themselves metabolites, which have the capacity to influence the 
_ metabolism of some, many, or of all the tissues with which they 


university OF ; 
LUNOIs LIBRAR 


352 RIDDLE: CONTROL OF SEX RATIO 


come in contact or which they may reach indirectly. A par- 
tial list of the animal forms that have been most studied in this 
respect is written serially on the top of our diagram—in a 
position intermediate to egg and adult. The number of these 
animal forms might be much increased, and the names of the 
investigators of this aspect of the modification of sex are quite 
too numerous” to be mentioned here. But the present point 
of interest is that these results, as a whole, demonstrate that 
the extent of secual modification in the experimental animal is, 
general, in proportion to the immaturity of the treated animal. 
That is to say, the earlier the internal secretion of the gonad is 
supplied or withdrawn—the earlier the metabolic change is 
effected—the more profound is the sexual modification of the in- 
dividual. All this is of course clearly in conformity with the 
Law of Genetic Restriction—a principle of embryology that is 
true alike for all of the known characteristics of the organism. 

Of the several animals of the list we may here particularize 
concerning only two or three. The stag is a form that has long 
been known to show thus a considerable and beautiful. series of 
greater modification of antlers and other so-called secondary 
sexual characters, in correspondence with castration at earlier 
and earlier periods in the life of the animal. The free-martin 
—another Ungulate—is now known to exemplify a much ear- 
lier point at which the foreign internal secretion begins to act; 


1 That changes following the removal of gonads, ete., have for many years 
been recognized as connected with a changed metabolism may be illustrated 
from the following quotation from Marshall (’10). ‘The effects of castration 
indicate that an alteration in the metabolism, even in comparatively late life, 
may initiate changes in the direction of the opposite sex’’ (p. 658). 

12 The following partial references are suggested by the particular animals 
listed in the diagram: Stag, Darwin (1868); Caton (1881); Fowxmr (1894); 
Rorie (1900). Human, Heaar (1893); Spetuerm (1898); Hixmer and RenauLT 
(1906); C. Wanuacr (1907); TANDLER and Gross (1909). Sheep, SHarrock and 
SreviemMan (1904); Seniegman (1906); MarsHaLtt and Hammonp (1914). Guinea- 
pig, Bourn and Ancet (1903-9); Srernacu (1910-13). Pheasant, GurNEy (1888). 
Fowl and Duck, Darwin (1868); Gurney (1888); Foecrs (1903); SHarrock and 
SELIGMAN (1906-7); GoopALE (1910-16). Pigeon, RippLE (1914 a). Frog, Nuss- 
BAUM (1907); Prutteer (1907); Sreinacn (1910); G. SmirxH (1912). Inachus and 
Carcinus, Potts (1909); G. Smrru (1910-12). Free-martin, Linure (1916). Bo- 
nellia, BALTZER (1914). 


RIDDLE: CONTROL OF SEX RATIO 353 


and here, true to the rule that has been established elsewhere in 
all this general line of work, the resulting modification is cor- 
respondingly strong and striking. When, by whatever means, 
we effect a change in the metabolism (which is the essential 
thing) at a still earlier stage—in the egg-stage in our own and 
in some other experimental reversals of sex—then we obtain in- 
dividuals whose sexual nature is quite thoroughly reversed;'* in 
many cases completely so, and in still other cases with varying 
degrees of completeness. 

Baltzer’s (14) beautiful experiments with the worm Bonellia 
best illustrate this fact and show the several stages of modifica- 
tion not only in one and the same animal form but in the in- 
dividuals hatching from a single brood. Baltzer finds that when 
the larvae of this animal are hatched they are capable of becom- 
ing either males or females. If they happen to become attached 
to the proboscis of an adult female they become males; if they 
do not succeed in so attaching themselves they soon settle from 
the water into the sand or mud of the sea-bottom and there 
undergo, quite slowly, further development into females (almost 
exclusively). The plastic, reversible, quantitative nature of sex 
in this form was shown by this investigator in the following 
way: Some of the free-swimming “‘indifferent” larvae -were 
artificially helped to a connection with the proboscis of an adult 
female. Some of these were permitted to maintain this attach- 
ment for a very short period; others were removed at progres- 
sively longer periods, with the very significant result that prac- 
tically all stages of hermaphroditism were -produced. Those 
first removed becoming almost perfect females, others with 
longer and longer periods a attachment, becoming more and 
more perfect males. 

Now the conditions under which the two sexes are here de- 
veloped afford, in our own opinion, good reasons for believing 
that the larva is stimulated—through its contact with the living 


13 The observations of Steche (’12) on the relation of precipitin reactions to 
sex, as seen in the blood of insects are of much interest. This author thus finds 
that male and female of the same species present differences as great as do the 
males of two related species, or as do the females of related species, 


354 RIDDLE: CONTROL OF SEX RATIO 


tissue—to a higher metabolism; supporting this point of view is 
the observed fact that ‘‘differentiation”’ is much hastened in this 
male individual as compared with the otherwise wholly similar 
larva that is destined to become a female. 

What it has been our privilege and opportunity to present is 
in itself but an outline or summary of result obtained in the 
modification and control of sex, and of the conclusions that 
seem to follow from these results. In a closing statement, there- 
fore, we wish only to direct attention to some consequences of 
the new knowledge of sex. As a foreword to this statement, 
however, we would note that not only do the widely different 
kinds of fact to which we have made reference directly support 
the view of the basis of sex here presented, but that nothing 
known of the sex-chromosomes is necessarily opposed to this view 
although an abundance of the data here presented sharply op- 
pose the conception that the sex-chromosomes are a cause of sex, 
or that they are even a necessary associated phenomenon. We 
may conceive that sexually differentiated organisms, from the 
first, have had the problem of prodveing germs pitched at two 
different metabolic levels; and if two sharply opposed sexes are 
to result from these two kinds of germs then the two metabolic 
levels must be measurably distinct. This task of producing and 
maintaining two kinds of cells pitched at two different levels 
ultimately falls upon cells, and these have, sometimes at least, 
produced two different chromosome complexes in connection with 
or in accomodation to the establishment of these two metabolic 
levels. Put, as we have seen, the requisite metabolic level of 
the germ may be established in the absence of the appropri- 
ate chromosome complex, and the sex of the offspring made to 
correspond with the acquired grade or level of metabolism. 

With these facts concerning the functional basis of sex in 
mind, and reverting to our first quotation from Doncaster, 
how little wonder that sex (despite its seeming “lack of func- 
tion’ is ‘“‘nearly universally distributed,’ almost coequal with 
‘“‘the fundamental attributes of living matter, irritability, as- 
similation and growth?” Since some grade of metabolism is of | 
necessity universally present in living matter the basis for two 


RIDDLE: CONTROL OF SEX RATIO 355 


sexes is of equally wide distribution in that sexual differentiation 
results from metabolic differentiation, through the establish- 
ment of two relatively distinct and relatively stable levels of 
metabolism. In the same way is accounted for the hitherto 
puzzling fact that the two sexes must have originated many 
times, scores, hundreds, or thousands of times, within species 
previously unisexual, during the long period involved in the 
evolutional history of organisms. 

Most important of all, perhaps, is the demonstration that one 
hereditary character is modifiable, is of a fluid, quantitative, re- 
versible nature. Seemingly this can only mean that other heredi- 
tary characters are also modifiable. The methods and results of 
most studies in modern genetics have asked us to accept a quite 
different view, namely that no such thing as control of heredity 
may be hoped for, but that we can only look to a sorting and 
elimination of germs, or of so-called hereditary factors—and to 
fortuitous origins or recombinations of the latter—to give us 
better or more desirable organisms. Surely there is a lot of 
fatalistic philosophy in that conception. All other aspects of 
function in biology recognize—and some have already attained 
—the control of life-processes as their aim and goal. Only in 
this field of heredity—involving the overwhelmingly important 
processes of continuance and of becoming—has this aim been 
accepted by a great and growing body of workers as impossible. 
If sex has been in fact controlled, if it has a modifiable metabolic 
basis—as now seems assured—then the life processes involved in 
heredity like other life-processes, invite the investigator to his 
full and complete task; territory hitherto labelled “impossible’”’ 
is open to investigation. 


BIBLIOGRAPHY 


Bayrzer,.F. Arch. f. Zellforsch., vol. 2, 1909. 

Baurzmr, F. Mitteil. Zool. Stat. Neapel., vol. 22, 1914. 

Banta, A.M. Year Book, Carnegie Inst. Wash., 1915. (Also Proc. Nat. Acad. 
Sci., vol. 2, 1916.) 

Bruu, A. G. Quoted from Popenoe. Jour. Hered., vol. 5, p. 47, 1914. 

Benepict, F. G., and Emmes, L. E. Jour. Biol. Chem., vol. 20, 1915. These 
authors give full references to the earlier literature. 


Reprinted from the JouRNAL OF THE WASHINGTON ACADEMY OF SCIENCES 
Vol. VII, No. 12, June 19, 1917 


GENETICS.—The réle of selection in evolution... W. EK. Castie, 
Bussey Institution. 


Up to the year 1900 those who believed in organic evolution 
almost without exception believed in selection as its efficient 
cause. Then came a period of doubt, maugurated by DeVries’ 
Mutation Theory and strongly supported by Johannsen’s Pure 
Line Theory. In the minds of many biologists at the present 
time selection is an obsolete agency in evolution and an 
adequate explanation of evolution is to be found only in muta- 
tion and pure lines. I believe this to be a mistaken view, not 
because mutation and pure lines are false, but because their 
applicability is very limited as compared with the broad field of 
organic evolution. To universalize them is to hide the world 
by holding a small object close to the eye. For even if we con- 
cede the strongest possible claim for mutation as an agency in 
evolution, viz, that it produces all new and heritable variations, 
it is still unable to produce evolution without the aid of selec- 
tion. The production of new variations produces no racial 
change unless those variations persist, but their persistence 
depends wholly upon selection. This is admitted by DeVries, 
the author of the mutation theory, but overlooked by many 
of those who have adopted the term mutation, as a scientific 
shibboleth. 

But it is idle to enter upon a discussion of either selection or 
mutation without carefully defining these terms, since both 
are often used quite ambiguously, the latter in particular being 
used in several different senses, and so being a cause of 
misunderstanding where no genuine difference of view exists. 

Ever since DeVries’ original attack in 1900, it has become 
increasingly common among biologists to refer with disrespect 
to ‘Darwinian selection.” But Darwin understood by selec- 
tion any agency which would cause one organism to survive 
rather than another, and it is not clear that any theory of evo- 
lution can dispense with such an agency. Since more organisms 
are born than can survive, some must perish. In a state of ~ 


1A lecture delivered before the Washington Academy of Sciences, April 13, 
1917. 


370 CASTLE: ROLE OF SELECTION IN EVOLUTION 


nature, that is, in a state of affairs not actively controlled by 
man, those creatures survive which are ‘best adapted to their 
surroundings. This is what Darwin meant by ‘‘natural selec- 
tion.”’ Among organisms under the immediate control of man, 
as the cultivated plants and domesticated animals, where the 
determination of what individuals shall become parents rests 
with man, Darwin recognized the occurrence of ‘‘artificial 
selection.” 

Any legitimate attack on Darwin’s views of selection must 
deal either with natural selection or with artificial selection. 
But when ‘Darwinian selection” is mentioned as 4 term of 
reproach, the attack is really directed neither against natural 
selection nor against artificial selection, nor against any other 
conceivable form of selection, but against one of Darwin’s views 
as to the nature of variability. Darwin recognized two sorts of 
heritable variations, (1) those which are purely quantitative, 
plus or minus, as compared with the prevailing racial condition, 
and (2) those which are wholly different from the prevailing 
condition. The former we may call “fluctuations,” adopting 
the convenient term of DeVries. The latter Darwin often 
called ‘‘sports.”’ Bateson has called them discontinuous varia- 
tions, and DeVries calls them mutations. Darwin believed that 
evolution might result either from the systematic and repeated 
selection of fluctuations or from the propagation of sports. 
DeVries doubts whether the systematic selection of fluctuations 
amounts to much in an evolutionary way, and Johannsen has 
denied to it any evolutionary effect whatever, on the ground 
that fluctuations are not inherited. Darwin assigned to the selec- 
tion of fluctuations a major part in evolution, DeVries assigned to 
it a minor part, and Johannsen allows it no part in evolution. 
As regards sports, Darwin assigned to their selection a minor 
part in evolution (chiefly among cultivated plants and domestic 
animals); DeVries ascribed to a particular kind of sports (his 
‘‘mutations”) a major part in evolution; and Johannsen ascribes 
_ an exclusive part in evolution to a type of variation which 
would include both Darwin’s sports and DeVries’ mutations 
and then some. Johannsen has indeed made a new classification 


CASTLE: ROLE OF SELECTION IN EVOLUTION 371 


of variations which is both logical and sound, but which has 
resulted in some confusion owing to efforts to combine it with 
earlier classifications. He classifies variations into those which 
are inherited (genotypic) and those which are not inherited 
(phenotypic). No objection can be made to this classification 
except that it raises new difficulties and solves none. For how 
is one to distinguish a phenotypic from a genotypic variation? 
Only by trying them out. A variation which is inherited is 
genotypic; one which is not inherited is phenotypic. Since 
there is no other way then actual experiment by which to dis- 
tinguish genotypic from phenotypic variations, we acquire only 
a new set of synonyms for inherited and non-inherited, a thing 
for which there was no urgent need. 

Attempts to combine the classifications of variations made re- 
spectively by Darwin, by DeVries, and by Johannsen have re- 
sulted in serious confusion which is largely responsible for the 
apparently contradictory views held at present concerning selec- 
tion. There really is no diversity of view concerning selection 
but only concerning the nature of the material that it acts upon 
(viz., variations). 

To complicate the situation still farther we have the discovery 
of Mendelian unit-characters which introduces a new un- 
certainty. Are these unit-characters fluctuations or sports? 
Do they arise solely by mutation or also by the cumulation of 
fluctuations? These are vital but perplexing questions. As 
matters stand concerning terminology, we have the term 
“sport,” introduced by Darwin but now largely discarded, 
meaning any discontinuous, striking, suddenly appearing varia- 
tion, known to be strongly inherited. Some of the examples 
_ cited by Darwin, such as the Ancon sheep, obviously involve 
Mendelian unit-characters. 

The term mutant as used by DeVries signifies much the same 
as Darwin’s term sport but involves a particular conception of 
the circumstances and manner of its origin which is not involved 
in Darwin’s term. Some of DeVries’ mutants of the evening 
primrose involve Mendelian unit-characters, as for example 
his dwarf mutant (nanella), while others such as gigas do not, 


372 CASTLE: ROLE OF SELECTION IN EVOLUTION 


The latter involves a double representation of every chromo- 
some in the cell nucleus; the Jata mutant involves the presence 
of a single extra chromosome. What chromosome changes, if 
any, are involved in other of DeVries’ mutants which do not 
Mendelize is unknown. Morgan has shown that in Drosophila 
a unit-character change almost certainly involves a change in a 
definitely localized part of a single chromosome. But he ap- 
plies the term mutation to each unit-character variation of 
Drosophila, of which he has observed over a hundred. Some 
of these are not at all striking, involving only a slight change in 
the shape, size, venation, or carriage of the wing, which might 
easily be overlooked by the ordinary observer. Many of them 
also fluctuate. Hence it is obvious that Morgan’s use of the 
term mutation is very different from that of DeVries, its origina- 
tor. To Morgan, mutation as illustrated in Drosophila is 
simply change by a unit-character. With this conception of 
mutation, Morgan attempts to combine the genotype concep- 
tion of Johannsen. He regards unit-character variations as the 
only kind of genotypic variations and these as fluctuating (if 
at all) only through the interaction of other unit-characters, 
each one by itself being incapable of fluctuation. : 

It will be observed that as regards the term mutation, we 
have a very confused state of terminology which results in 
much discussion at cross-purposes, because persons using the 
same term have different things in mind. 

But in this discussion, however confused its terminology, 
there are really involved two contrasted sets of general ideas, 
two alternative lines of explanation of evolutionary change, 
one favored by Darwin, the other offered as a substitute by 
DeVries and accepted by Johannsen and Morgan. We may 
briefly outline them as follows: 


Darwin DEVRIES 
1. New types are for the most 1. New types are created only 
part created gradually. abruptly. 


2. New types are for the most 2. New types are fully stable. 
part plastic. 

3. One evolutionary change fol- 3. One evolutionary change has 
lows upon and is made pos- no necessary relation to 
sible by another. another. 


CASTLE: ROLE OF SELECTION IN EVOLUTION . 373 


4. Natural selection determines 


what classes of variations 
shall survive and, in conse- 
quence, what shall be the 
variable material subjected to 
selection in the next genera- 


4. Natural selection determines 


only what classes of varia- 
tions shall survive, and exer- 
cises no influence on the 
subsequent variability of the 
race. 


tion. 

5. The further evolution of our 5. Evolution is beyond our con- 
domestic animals and culti- trol except as we discover and 
vated plants (and of man isolate variations. 
himself) is to some extent 
controllable because we can 
by selection influence the va- 
riability of later generations. 


These two sets of contrasted views remind us somewhat of 
_the theological ideas of free-will and predestination respectively, 
which resemblance will account for the preferences of some biol- 
ogists but will not prove which is right and which is wrong. 
This is wholly a matter for evidence. But what conclusion one 
reaches will depend much upon what sort of evidence he studies. 
Paleontology, geographical distribution, classification, and experi- 
mental breeding, all present evidence which must be weighed 
before a safe verdict can be framed. 

Paleontology, the study of the actual historical records of 
evolution found in the rocks, indicates in the case of the most 
complete series of fossils, as for example of the horse, the camel, 
and the rhinoceros, that the evolution of these types was a 
gradual process, though of course their appearance in particular 
continents may have been abrupt, owing to migration. It 
indicates further that these and other types, when they first 
appeared, were plastic, and generalized and varied in many 
different ways, most of the variations later disappearing and 
leaving only a favored few lines of specialized survivors. It 
shows too that one variation paved the way to another. The 
five-toed horse first becomes four-toed, then three-toed, then 
one-toed. There is no mutation from five-toed to one-toed, 
nor from the size of a fox to that of a draft horse. As to natural 
selection, paleontology is silent, because the causes of extinction 
are unknown. But on the whole the weighty evidence of pale- 


374. CASTLE: ROLE OF SELECTION IN EVOLUTION 


ontology supports the view that evolution as an age long process 
has been gradual and progressive, not abrupt and unguided. 

Geographical distribution and classification favor the same 
idea. Related species are most often found in contiguous terri- 
tory. Species not closely related are commonly far separated 
in space or have been long separated in time. Nothing indicates 
that of two related species one has sprung suddenly from the 
other. They are not distinguished from each other, as a sport 
from its parent form, by some single Mendelian unit-character, 
but they differ morphologically by a large number of quantita- 
tive differences, and physiologically they differ to such an extent 
that frequently they will not interbreed when brought together 
even though their morphological differences are small, or they 
will produce sterile hybrids, or those of a blended, intermediate 
character. In all these particulars they show that they have 
not diverged by mutation, either in the sense of De Vries or in 
that of Morgan, but by a gradual progressive process. 

Finally we come to the evidence from experimental breeding. 
Some say that this is the only legitimate evidence as regards the 
method of evolution because it alone is experimental. I should 
be the last to deny its importance because I have devoted much 
time to its pursuit in the firm conviction that it could yield 
valuable evidence, but frankness compels one to admit that this 
method of study, like all the others, has limitations of its own. 
The experimental breeder can study a few successive generations 
with an intensiveness that is possible by no other method, but 
his glimpses of evolution at work are momentary as compared 
with the studies of the paleontologist. He can witness the pro- 
duction of new sorts but it is doubtful whether any man has 
witnessed the contemporary production of a new species, in the 
sense of the paleontologist and the student of geographical 
distribution. Evolution is undoubtedly at work all the time, 
but the breeder is not always in a position to say just what is 
happening. It takes a succession of views in a motion picture 
to show what objects are stationary and what are moving, and 
the breeder’s view of the evolutionary process often fails to 
reveal which is which. 


CASTLE: ROLE OF SELECTION IN EVOLUTION 375 


On the other hand, the experimental breeder, though he lacks 
perspective, is dealing with the actual material concerned in 
organic evolution. He can see and handle it and observe it 
change under his hands, as no other student of evolution can. 
But the changes which he observes taking place must be cor- 
rectly interpreted if valid conclusions are to be reached concern- 
ing the general process of evolution. At present experimental 
breeders are divided in their views. The very same facts are 
interpreted by some as indicating an orderly progress toward 
definite end results, and by others as nothing but haphazard 
unrelated chance occurrences. Just now the latter method of 
interpretation, embodied in the mutation theory, is very 
popular among experimental breeders, although it has few 
adherents among students of paleontology, classification, or 
geographical distribution. 

The principal tools of the experimental breeder are hybridiza- 
tion and selection. All are agreed that hybridization (using 
the term in its broadest sense) is, in the hands of the breeder, 
a very potent agency in producing variability, upon which selec- 
tion may then be brought to bear for the production of new or 
modified types. Lotsy even goes so far as to suggest that all 
genetic variability is the result of hybridization, but this is 
flatly disproved by observations of Johannsen who reports the 
occurrence of mutations in genotypically pure lines of beans, 
as also by the remarkable series of variations observed by Mor- 
gan in an inbred race of Drosophila. 

As regards the action of selection, the most widely divergent 
views are held by experimental breeders. The mutationists 
hold that it can do nothing but isolate variations which may 
sporadically put in an appearance or which may by hybridiza- 
tion be brought together into new combinations. Those who 
differ from them, and whom they call selectionists, maintain 
that selection can accomplish more than the mere isolation of 
variations because it can, by a series of selections, influence 
further variability. I confess myself an adherent of this at 
present somewhat unpopular view. I hold it, not because 
Darwin held it, nor merely because paleontologists, systematists, 


376 CASTLE: ROLE OF SELECTION IN EVOLUTION 


and students of geographical distribution in general favor it, 
nor because DeVries and Johannsen have attacked it, but be- 
_ cause the facts of experimental breeding, as I understand them, 
prove it. 

For DeVries may be claimed the merit of having first system- 
atically set about testing the effects of selection by actual experi- 
ment. His methodical selections for many years in succession 
of maize, buttercups, striped flowers, and four-leaved clover 
will long be remembered, but they fall far short of conclusiveness 
because they were not continued long enough to show whether 
selection had attained all that was attainable under existing 
variability or whether further variation in the direction of selec- 
tion would occur, and because DeVries’ cultures were not suffi- 
ciently guarded from hybridization which might conceivably 
influence the result. These necessary precautions were fully 
met by Johannsen, who in the case of beans, which are self 
fertilizing but show fluctuating variation in the size of the seed, — 
proved that selection generation after generation in a particular 
direction may be without result, so far as any change in average 
seed size is concerned. Cases of this sort involve “‘pure lines,” 
those which are devoid of genetic variation to any appreciable 
extent in the character studied, size of seed. But in other cases, 
as where Johannsen made his size selections from a field crop 
harvested from many different plants, he found that average size 
was influenced by selection, which he reasonably explains on the 
ground that the material from which selection was made con- 
sisted of a mixture of pure lines genetically distinct. The 
correctness of Johannsen’s conclusion has been repeatedly veri- 
fied in the case of other self-fertilizing plants such as wheat and 
oats. Attempts were at once made to generalize Johannsen’s 
brilliant demonstration of the principle of pure lines in the 
following ways: 

1. Since a line of beans long self-fertilized is devoid of genetic 
variation in seed size, self-fertilization, if long enough continued, 
will produce lines genetically pure as regards all characters. 
Selection can not bring about modification of such pure lines. 
In respect to this generalization it may be said that it remains to 


CASTLE: ROLE OF SELECTION IN EVOLUTION 377 


be shown that beans are as devoid of genetic variation in other 
particulars as they are in seed size, which the argument assumes 
to be true. Further, if various pure lines of beans have come 
into existence by an evolutionary process (descent from a common 
ancestor, with modification) it is evident that differences must 
have arisen which did not originally exist. Suppose we grant 
Johannsen’s (unproved) contention that such differences arise 
by mutation only. If they arise in this way (or in any other way 
whatsoever), selection can isolate them, and if they are at all 
frequent in occurrence, selection can be continuously effective 
in producing racial changes. It would all come down then toa 
question of how frequent mutations are in a particular case. 
Johannsen concedes their occurrence even in beans. It may 
well be that in some organisms they are commoner than in others 
and that in beans they happen to be particularly infrequent. 

2. Johannsen’s case has been further generalized to include 
all self-fertilizing organisms, which are supposed to fall auto- 
matically into pure lines (i.e., those devoid of genetic variation) 
as regards all characters. This too requires proof, but has been 
found to be a safe working hypothesis in the case of cereals, 
tobacco, peas, and other economic crops, in the attempted im- 
provement of which selection of fluctuations, unless preceded 
by hybridization may be regarded as a waste of time, for the 
reason that genetic variation is so rare under continuous self- 
fertilization that the breeder will obtain variation much more 
quickly by resorting to hybridization. 

3. Further, it has been argued that if cross fertilization alone 
interferes with the automatic production of pure lines, then any 
organism which dispenses with fertilization altogether, reproduc- 
ing asexually, must zpso facto constitute a pure line. Jennings 
sought to test out, this conclusion by experiment. He selected 
size variations in Paramecium which reproduces by fission, 
with success in the case of mass cultures of unknown origin, 
but without success in the case of cultures made from single 
individuals. This was regarded as strong confirmation of the 
pure line principle until Calkins and Gregory, repeating the 
experiment on ex-conjugants, were unable to support it. Then 


378 CASTLE: ROLE OF SELECTION IN EVOLUTION 


Jennings, selecting a new species of Protozoa, more favorable 
for precise quantitative observation, also obtained a different 
result. He now found that among the observed fluctuations in 
size, those of a genetic character were included, so that by 
repeated selection races could be produced which were progres- 
sively larger or smaller, rougher or smoother. This is fully im 
harmony with the observations of Stout who found that varia- 
tions in Coleus arising in asexual propagation were capable of 
further propagation. It also harmonizes with the observation 
of Shamel as regards the occurrence in citrous fruits of bud 
variations which are important enough to warrant propagation in 
economic work; and further, with Winkler’s clear demonstration 
of the occurrence in the tomato and the night-shade of gigas like 
mutations, arising first in single somatic cells, which asexually 
propagated produce entire plants of a new type which then are 
self-perpetuating by seed. We also have the observations of 
East that in the asexual propagation of the potato occasional 
bud variations may occur which are similar in nature to unit- 
character variations in reproduction by seed. It is accordingly 
clear that the pure-line principle does not apply without excep- 
tion to asexually reproducing organisms any more than it does 
to self-fertilizing ones. It is true, however, that genetic varia- 
tions are much less common among such organisms than among 
those produced by cross-fertilization. Herein lies the justi- 
fication of present agricultural practice in the breeding of self- 
fertilized cereals, and of horticultural practice in the propagation 
by grafts, runners, layers, etc., of superior individual plants. 

4. Attempts to extend the pure line principle to organisms 
which are not self-fertilizing (and this includes all the domestic 
animals and many cultivated plants) have met with small 
success. Morgan indeed assumes that it applies to his races of 
Drosophila up to a certain point, the point at which mutation 
begins, but the mutations which he recognizes are so numerous, 
so minute in many cases, and so fluctuating in others, that it 
becomes a question whether his “mutations” are not just ordi- 
nary heritable variations. Morgan would undoubtedly admit 
this since he claims that all heritable variations arise as mutations, 


CASTLE: ROLE OF SELECTION IN EVOLUTION 379 


but this is simply juggling with names, giving a new meaning 
to the word mutation in order to justify a sweeping generaliza- 
tion otherwise untenable. 

The test of a pure line is its freedom from any genetic varia- 
tion, so that selection cannot modify the racial mean as regards 
any character. As soon as any race of animals or plants changes 
in response to selection, it must be forthwith excluded from the 
category of pure lines. The consequence is that no case of a 
pure line among animals has yet been demonstrated. Never- 
theless the “principle of the pure line” is in some way or other 
supposed by the followers of Johannsen to confer on even the 
higher animals a limited liability to modification in consequence 
of selection. 

Thus Pearl having been entrusted in 1908 with a selection 
experiment for increase of egg production in Plymouth Rock 
fowls, an experiment which had already been in progress for 
nine years, decided after a study of the records kept by his 
predecessor that no improvement whatever had up to that time 
been made and further that none probably could be made since 
individual wild birds probably lay, under favorable conditions, 
as many eggs as their best tame relatives. This reasoning was 
strictly in accordance with the ‘“‘pure line principle”’ and was in 
fact based on it. 

Later by changing somewhat the basis of selection, so as to 
rank his animals on the basis of their progenies’ performance as 
well as their own, Pearl found that he could considerably in- 
crease the flock average. Yet he still maintains that he has 
only more good birds not better ones, than when the experiment 
began, and in loyalty to the pure line principle he has no expecta- 
tion of obtaining better ones in the future, since he already has 
and has had all along the ne plus ultra sort. One less devoted 
than Pearl to a generalization of the pure Jine doctrine would 
continue hopefully the effort to produce a better fowl as well as 
to produce more good ones. For the function of egg-production 
admittedly depends upon many physiological factors (as well as 
several external ones). These physiological factors must many 
of them be independently variable and to some extent independ- 


380 CASTLE: ROLE OF SELECTION IN EVOLUTION 


ently heritable. Variation in one or more of these factors (by 
mutation or otherwise) would undoubtedly influence the total 
productiveness, and the probability of the occurrence of a muta- 
tion would increase with the number of factors involved. So 
that even one formally committed to the pure line doctrine, but 
admitting as Johannsen does that mutations do occasionally 
occur in pure'lines, might hopefully continue to look for improve- 
ment in the standard of egg-production. No other method of 
detecting and utilizing a favorable variation, when it does occur, 
can be suggested than the very method of methodical and per- 
sistent selection against which the pure line advocates direct 
such vigorous attacks. 

Morgan is a formal adherent of the pure line doctrine, but 
pragmatically a selectionist for he admits the great progress 
made in the improvement of domestic animals and plants by 
selection, and even that his own mutants of Drosophila fluctuate 
and yield modified forms in response to methodical selection, 
as for example the bar-eyed mutant, subjected with success to 
plus and minus selection by Zeleny. But he attempts to explain 
these results in harmony with the pure line principle by assum- 
ing that, whenever a modification is observed in any character, 
this is due to a mutation, and if a graded series of modifications 
is obtained, as in the plus and minus selected bar-eyed Droso- 
phila, this is due to a multiplicity of mutating factors whose 
action on the chief factor concerned is purely incidental.. On 
this view, however, the attainment of a completely homozygous 
condition on the part of all factors (if all are indeed Mendelian) 
would put an end to genetic variability, and selection would 
then cease to produce effects. Such a completely stable con- 
dition has, however, rarely been demonstrated. One case is 
reported by MacDowell, that of a race of Drosophila with an 
extra number of thoracic bristles. The average number of 
bristles was increased by selection for six generations but then 
showed no further increase and could not subsequently be 
changed either upward or downward by further selection. The 
race had apparently become a ‘‘pure line” for bristle number. - 


CASTLE: ROLE OF SELECTION IN EVOLUTION 381 


In the case of certain characters in guinea-pigs I have re- 
peatedly attempted modification of a racial character by selec- 
tion within an inbred race, without success. Thus a very dark 
form of Himalayan albino, after a certain amount of improve- 
ment by selection, could not be further darkened to any appreci- 
able extent. A race selected simultaneously for large size and 
for small size showed so little change that the experiment was 
abandoned after a few generations. No indication was forth- 
coming that we could thus ever approach in size either the small 
wild Cavia Cutleri of Peru, or the large races of guinea-pig kept 
in captivity by the natives of the same region. Yet evolution 
had in some way evidently produced these divergent conditions 
from a single original source. The changes were probably too 
slow to be observable in the life time of one observer. 

On the other hand, certain characters of guinea-pigs, rabbits, 
and rats have been found to respond readily to selection in a 
particular direction. This is notably true of color patterns 
which involve white spotting. A selection experiment with 
hooded rats selected simultaneously in plus and minus directions 
has produced one race which is black all over except a white 
patch of variable size underneath, and another race which is 
. white all over except for the top of the head and the back of the 
neck, which are black. The races do not overlap at all and have 
not done so for many generations, though they still continue to 
diverge from each other as a result of continued selection. 

In similar experiments with Dutch marked rabbits it has been 
found possible by selection to increase or decrease the amount of 
white at will. Ina series of such rabbits ranging from nearly 
all black to nearly all white, stages far enough apart to be cer- 
tainly identifiable behave as Mendelian allelomorphs in crosses, 
but regularly emerge from such crosses in a slightly modified 
form, the whiter stages having been darkened and conversely 
the darker stages whitened. The principle of the pure line 
manifestly does not apply to these cases. White spotting is 
apparently a character which from its nature fluctuates con- 
stantly, such fluctuations having, to some extent at least, a 
genetic basis, since continuous selection invariably produces a 


382 CASTLE: ROLE OF SELECTION IN EVOLUTION 


modified race. Even in wild species, such as the skunks, white- 
spotting is manifestly a variable character, which no doubt will 
respond to the selective efforts of our skunk farmers, who de- 
sire an all-black race. Why white-spotting should be a less 
stable character genetically than some others, it is impossible 
to say, but the fact is beyond question. Morgan has suggested 
that in general the genetic basis of a Mendelian character may 
be a single molecule, and gives this as a reason for believing in its 
constancy. But white spotting can hardly fall in with this 
conception. It seems to me more probably due to a quantitative 
deficiency in the germ of some substance which normally finds 
its way into all epidermal cells of the body and which is responsible 
for the development in them of melanin pigment. Greater and 
greater deficiencies of this substance cause more and more 
extensive white areas. 

Complete or total albinism behaves. very differently. It 
results from a complete change in some color factor whichmay 
well be a simple molecule since it appears to be incapable either 
of contamination in crosses or of modification under selection. 
Nevertheless the color factor (molecule or whatever it may be) 
evidently is not so simple but that it can assume at least four 
mutually allelomorphic forms, as shown for the guinea-pig by 
Wright, a like number of allelomorphs, though not their exact 
equivalents, being known also in the rabbit. 

As regards the agouti factor in mice, rabbits, and guinea-pigs, 
this too may assume several different allelomorphic conditions, 
though it is not certain that any one of these fluctuates or can 
be modified other than by associating with it unrelated genetic 
factors. 

The divergent conclusions which students of genetics have 
reached concerning the stability of Mendelian genes and the 
consequent effects of selection for their modification are probably 
due in part to the particular choices which they have made of 
test cases. A study of albinism alone would lead one to believe 
in the fixity and constancy of Mendelian genes and the impossi- 
bility of modifying them by selection. A study of white spotting 
leaves one with the unshakable conviction that this form of 


CASTLE: ROLE OF SELECTION IN EVOLUTION 383 


gene is plastic and yields readily to selection. Where only 
genes of the former sort are involved, the principle of the pure 
line is applicable; where genes of the latter sort are involved, it 
is not applicable. The divergent results obtained by Jennings 
when dealing with Paramecium and when dealing with Difflugia 
indicate that among asexually reproducing organisms, also, genes 
are involved, some of which are stable, some of which are not. 
Accordingly, what conclusion we reach as to the applicability of 
the pure line theory in the breeding of animals and plants will 
depend upon how common we find stable and plastic genes 
respectively to be, and in what sorts of variations they are 
involved. 

My own opinion, based upon a study through many years of a 
variety of inherited characters in the smaller mammals, inclines 
to the view that in such animals very few characters can safely 
be referred to the agency of perfectly stable genes. Even in 
color characters, probably the simplest as well as the most 
studied of inherited characters, there is much fluctuation which 
yields substantial results to selection by the discriminating 
breeder. The yellows are not all of one shade, nor the blacks of 
equal depth. The golden yellow of the Guernsey cow is very 
different from the fawn of the Jersey or the dark red of the Devon. 
Yet all are yellows, allelomorphs of black, but each is selected for 
a different standard to which the breeder must adhere very 
carefully in his selections, if he wishes to win prizes or sell breed- 
ing stock. 

When it comes to size and shape and that consistent inter- 
relation of parts which the breeder calls ‘‘conformation,” stable 
genes cannot be detected. Crosses produce blends as regards 
size and shape, and conformation is completely dissipated by a 
cross. ‘That is why the breeder is so reluctant to resort to an 
outcross unless he is engaged merely in meat or wool production 
and is not attempting to breed to a type. Aside from color there 
are very few valued economic characters in our domestic animals 
which are not inherited after the manner of blends. 

Weight of carcass, quality of wool, milk production in cattle, 
egg production in fowls—all these are blending characters which 


384 CASTLE: ROLE OF SELECTION IN EVOLUTION 


in later generations show either no segregation or imperfect 
segregation (fowls, Pearl?). JI do not say that in these cases no 
Mendelian inheritance is involved, but merely that no stable 
genes are in evidence, nothing that would preclude the probable 
effective use of selection in maintaining or raising breed standards. 

If we turn from the breeding of animals, in which manifestly 
the pure line principle has little applicability, to the breeding 
of plants other than those which are self-fertilized, we again 
find that this principle has a very limited applicability. Prob- 
ably the most valuable open pollinated field crop in cultivation 
is maize. But a pure line of maize is not known to exist. An 
experiment which should have lead to the production of pure 
lines, if such a thing were attainable in maize, has been in prog- 
ress at the University of Illinois for the past twenty years. 
Selection has been made for increased and for decreased protein 
content of the grain, and also for both increased and decreased 
oil-content, with the result that steady progress in the direction 
of selection has in every case been made. The high protein 
strain now contains twice as much protein as the low protein 
strain; and the high oil strain contains four times as much oil 
as the low oil strain. The divergence of the selected lines from 
each other is not now as rapid as at first but it still continues 
steadily, with no indication that it is soon to cease, as must be 
the case if only stable genes were involved. 

Those characters in maize which directly affect the yield, 
such as size of plant, or of the grain which it bears, are blending 
in inheritance and show imperfect segregation subsequently. 
They are probably all of them quite as amenable to selection as 
the oil content and protein content of the seed, experimented 
upon in Illinois. 

>It is true that Pearl (1912) has described fecundity in fowls as ‘“‘typically 
Mendelian” in heredity but his figures show that in crosses between Barred 
Rocks and Cornish Indian Games, the average fecundity of the F, birds is in 
both the reciprocal crosses intermediate between that of the respective parent 
races though nearer the racial average of the sire, which supports his contention 
that a sex-linked gene is involved, but shows also that this is not the only factor 
involved. Back-crosses of Fi of both sexes with the pure races give evidence of 


further blending (or imperfect segregation) on the part of the non-sex-linked 
factor or factors. 


CASTLE: ROLE OF SELECTION IN EVOLUTION 385 


Finally, as evidence that even in self-fertilized plants the pure 
line principle may be inapplicable because of the existence of 
genes which-are plastic, let me cite a very extensive and care- 
fully executed piece of work on garden peas done by Hoshino. 
He studied the behavior of flowering time, and showed that its 
inheritance involves a Mendelian gene coupled with flower 
color (white or red). The inheritance of flowering time is inter- 
mediate, but F, is closer to the late than to the early parent in 
this character. Segregation is imperfect in F, with a range 
practically all the way from the early to the late parent, but not 
transgressing this range. F,; and F, families from self-fertilized 
parents are in many cases quite variable but others are no more 
variable than the pure parental varieties and so may be treated 
as practically “constant.” A study of the average flowering 
time of each of the 230 “constant”? F, families shows that these 
fall into three main groups, some falling into a modified early 
group, not quite so early as the early parent, others falling into a 
modified late group, not quite so late as the original late parent, 
' but most of all falling into an intermediate group occupying the 
region midway between the parent varieties in flowering time. 
Considered all together, the F, families ‘‘constant”’ for flowering 
time form an almost uninterrupted series of conditions con- 
necting the respective parental conditions seen in the early 
flowering and in the late flowering race. 

These observations show the existence of a gene for flowering 
time in peas which is decidedly plastic. That a gene actually 
exists is shown by its coupling with flower color. That it is 
plastic is shown by the fact that it emerges from the cross nearly 
always in a modified form. When the possibility of modifica- 
tion has been continued as long as the F, generation, the majority 
of the “‘constant”’ families are found in the intermediate or middle 
group. The plasticity is here shown in a tendency of the con- 
trasted genes to blend into one of intermediate character. It 
is also shown in data given by Hoshino as to flowering time in 
parent individuals and their offspring in the late flowering variety. 
Although this variety is treated by Hoshino as a “pure line,” 
it is evident that within this line itself the later flowering in- 


386 CASTLE: ROLE OF SELECTION IN EVOLUTION 


dividuals have later flowering offspring and vice versa. In other 
words selection within this supposed “pure line” is evidently 
effective. Accordingly either the gene here involved is plastic 
or the supposed pure line is not pure. 

From the various lines of evidence which have been cited (and 
I might have cited many more) it is clear that the pure line prin- 
ciple, valid as a working hypothesis for seed size in beans and 
for certain morphological characters in self-fertilized cereals, 
does not fit in with the observed facts as regards the effects 
of selection in the majority of the domesticated animals and 
cultivated plants, nor even with the behavior of certain characters 
in self-fertilized plants and asexually propagated animals. In 
the case of such characters as white spotting in mammals, it is 
evident that a change in the mean of the character in a partic- 
ular direction in consequence of selection actually displaces in 
the direction of selection the center of gravity of variation, so 
that in a very true sense selection makes possible further varia- 
tion in that same direction. The same is probably true as re- 
gards protein content and oil content in the Illinois corn experi- 
ment. It is doubtful whether, outside of that particular experi- 
ment, maize with as high a protein content as 15 per cent has 
ever been observed, or maize with as high an oil content as 8.5 
per cent. It is not then a misuse of terms to say that the selec- 
tion has in this case been the cause of further variation in the 
direction of selection and so an agency in the progressive evolu- 
tion of a new type. 

If this is true concerning a single character under experimental] 
study for a period of twenty generations, may it not also be 
true of entire organisms and groups of organisms subjected to 
keen competition with all other organisms in a struggle for 
existence which has continued for millions of generations? If 
there are characters which are plastic under artificial selection, 
why need we be skeptical about the plasticity of organisms sub- 
jected to natural selection? If artificial selection can, in the 
brief span of a man’s life time, mould a character steadily in a 
particular direction, why may not natural selection in unlimited 
time also cause progressive evolution in directions useful to the 


CASTLE: ROLE OF SELECTION IN EVOLUTION 387 


organism? Iam not ready to say that natural selection is proved 
as the method par excellence of evolution, but I am not ready to 
abandon it as the most reasonable explanation of evolution 
until something better supported than the mutation theory is 
offered as a substitute for it. At the same time the fact should 
be emphasized that biology has benefited greatly from the 
investigation and the discussion initiated by the mutation 
theory. Even though the mutation theory cannot be accepted 
as a general theory of evolution it has done us great good in 
dispelling or clarifying the hazy notions which formerly existed 
as to what natural selection could accomplish. Selection, whether 
natural or artificial, is, as the mutation theory rightly holds, 
primarily an agency for the elimination of variations, not for 
their production. It can only act on variations actually exist- 
ing, and while it can, I believe, continue and extend variation 
already initiated by shifting in the direction of selection the 
center of gravity of variation, it cannot initiate new lines of 
variation. It cannot change a vertebrate into something else 
nor something else into a vertebrate. It is limited to the modi- 
fication of existing types of organisms, and to their modification 
in directions in which they show a tendency spontaneously to 
vary. 
BIBLIOGRAPHY 


Castin, W. H., 1916. Genetics and Eugenics. Harvard Univ. Press. 

Hosuino, Y., 1915. On the inheritance of the flowering time in peas and 
rice. Journ. Col. Agr. Tohoku Imp. Univ., vol. 6. 

JOHANNSEN, W., 1909. Elemente der exakten Erblichkeitslehre. Jena. 

MacDowe tu, E. C., 1915. Bristle inheritance in Drosophila. Journ. Exp. 
Zool., Vol., 15. 

Morean, T. H., 1916. A critique of the theory of evolution. Princeton 
Univ. Press. 

Peart, R., 1912. The mode of inheritance of fecundity in the domestic fowl. 
Journ. Exp. Zool., vol. 13. 

Peary R., 1916. Fecundity in the domestic fowl and the selection problem. 
Amer. Ass. Nat. Vol. 50. 

Scorr, W. B., 1917. The theory of evolution. New York. 

Smira, L. H., 1912. Altering the composition of Indian corn by seed selection. 
Journ. Indust. Eng. Chem., vol. 4. 

DrVrizs, H., 1900-1903. Die Mutationstheorie. Leipzig. 

Wricut, 8., 1916. Studies of inheritance in guinea-pigs and rats. Carnegie 
Inst. Wash., Pub. 241. 


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